[xabt] Fix NRT annotations in Xamarin.Android.Build.Tasks, part 4 (#10364)

Context: #10326

This PR enables nullable reference types (NRT) for two files in the
`src\Xamarin.Android.Build.Tasks\Utilities` directory, following the
repository's NRT guidelines.

## Changes Made

**JavaResourceParser.cs:**

- Enabled `#nullable enable`
- Made `CodeTypeDeclaration` non-nullable throughout for cleaner API
  design
- Updated `Parse` method to always return a valid
  `CodeTypeDeclaration` object instead of null
- Removed unnecessary null checks in lambda functions and used
  null-coalescing operators
- Simplified calling code in `GenerateResourceDesigner.cs` by
  eliminating null handling
- Maintained Log property validation for safety

**MD2Managed.cs:**

- Enabled `#nullable enable` 
- Fixed `Padding` method to throw `ArgumentException` for invalid
  input instead of returning null
- Preserved existing cryptographic logic while ensuring type safety

Co-authored-by: Jonathan Peppers <[email protected]>

Author: Copilot

src/Xamarin.Android.Build.Tasks/Utilities/JavaResourceParser.cs

@@ -1,5 +1,4 @@
-#nullable disable
-
+#nullable enable
 using System;
 using System.CodeDom;
 using System.Collections.Generic;
@@ -18,7 +17,10 @@ public CodeTypeDeclaration Parse (string file, bool isApp, Dictionary<string, st
 			if (!File.Exists (file))
 				throw new InvalidOperationException ("Specified Java resource file was not found: " + file);
 
-			CodeTypeDeclaration resources = null;
+			if (Log == null)
+				throw new InvalidOperationException ("Log property must be set before calling Parse");
+
+			CodeTypeDeclaration? resources = null;
 
 			using (var reader = File.OpenText (file)) {
 				string line;
@@ -33,12 +35,12 @@ public CodeTypeDeclaration Parse (string file, bool isApp, Dictionary<string, st
 				}
 			}
 
-			return resources;
+			return resources ?? new CodeTypeDeclaration ("Resource") { IsPartial = true };
 		}
 
-		static KeyValuePair<Regex, Func<Match, bool, CodeTypeDeclaration, Dictionary<string, string>, CodeTypeDeclaration>> Parse (string regex, Func<Match, bool, CodeTypeDeclaration, Dictionary<string, string>, CodeTypeDeclaration> f)
+		static KeyValuePair<Regex, Func<Match, bool, CodeTypeDeclaration?, Dictionary<string, string>, CodeTypeDeclaration>> Parse (string regex, Func<Match, bool, CodeTypeDeclaration?, Dictionary<string, string>, CodeTypeDeclaration> f)
 		{
-			return new KeyValuePair<Regex, Func<Match, bool, CodeTypeDeclaration, Dictionary<string, string>, CodeTypeDeclaration>> (new Regex (regex), f);
+			return new KeyValuePair<Regex, Func<Match, bool, CodeTypeDeclaration?, Dictionary<string, string>, CodeTypeDeclaration>> (new Regex (regex), f);
 		}
 
 		// public finall class R {
@@ -49,11 +51,11 @@ static KeyValuePair<Regex, Func<Match, bool, CodeTypeDeclaration, Dictionary<str
 		//     }
 		//   }
 		// }
-		List<KeyValuePair<Regex, Func<Match, bool, CodeTypeDeclaration, Dictionary<string, string>, CodeTypeDeclaration>>> Parser;
+		List<KeyValuePair<Regex, Func<Match, bool, CodeTypeDeclaration?, Dictionary<string, string>, CodeTypeDeclaration>>> Parser;
 
 		public JavaResourceParser ()
 		{
-			Parser = new List<KeyValuePair<Regex, Func<Match, bool, CodeTypeDeclaration, Dictionary<string, string>, CodeTypeDeclaration>>> () {
+			Parser = new List<KeyValuePair<Regex, Func<Match, bool, CodeTypeDeclaration?, Dictionary<string, string>, CodeTypeDeclaration>>> () {
 			Parse ("^public final class R {",
 					(m, app, _, map) => {
 						var decl = new CodeTypeDeclaration ("Resource") {
@@ -73,6 +75,7 @@ public JavaResourceParser ()
 					}),
 			Parse ("^    public static final class ([^ ]+) {$",
 					(m, app, g, map) => {
+						g ??= new CodeTypeDeclaration ("Resource") { IsPartial = true };
 						var t = new CodeTypeDeclaration (GetNestedTypeName (m.Groups [1].Value)) {
 							IsPartial       = true,
 							TypeAttributes  = TypeAttributes.Public,
@@ -85,8 +88,9 @@ public JavaResourceParser ()
 					}),
 			Parse (@"^        public static final int ([^ =]+)\s*=\s*([^;]+);$",
 					(m, app, g, map) => {
+						g ??= new CodeTypeDeclaration ("Resource") { IsPartial = true };
 						var name = ((CodeTypeDeclaration) g.Members [g.Members.Count-1]).Name;
-						var f = new CodeMemberField (typeof (int), ResourceIdentifier.GetResourceName (name, m.Groups[1].Value, map, Log)) {
+						var f = new CodeMemberField (typeof (int), ResourceIdentifier.GetResourceName (name, m.Groups[1].Value, map, Log!)) {
 								Attributes      = app ? MemberAttributes.Const | MemberAttributes.Public : MemberAttributes.Static | MemberAttributes.Public,
 								InitExpression  = new CodePrimitiveExpression (ToInt32 (m.Groups [2].Value, m.Groups [2].Value.IndexOf ("0x", StringComparison.Ordinal) == 0 ? 16 : 10)),
 								Comments = {
@@ -98,8 +102,9 @@ public JavaResourceParser ()
 					}),
 			Parse (@"^        public static final int\[\] ([^ =]+) = {",
 					(m, app, g, map) => {
+						g ??= new CodeTypeDeclaration ("Resource") { IsPartial = true };
 						var name = ((CodeTypeDeclaration) g.Members [g.Members.Count-1]).Name;
-						var f = new CodeMemberField (typeof (int[]), ResourceIdentifier.GetResourceName (name, m.Groups[1].Value, map, Log)) {
+						var f = new CodeMemberField (typeof (int[]), ResourceIdentifier.GetResourceName (name, m.Groups[1].Value, map, Log!)) {
 								// pity I can't make the member readonly...
 								Attributes      = MemberAttributes.Public | MemberAttributes.Static,
 						};
@@ -108,6 +113,7 @@ public JavaResourceParser ()
 					}),
 			Parse (@"^            (0x[xa-fA-F0-9, ]+)$",
 					(m, app, g, map) => {
+						g ??= new CodeTypeDeclaration ("Resource") { IsPartial = true };
 						var t = (CodeTypeDeclaration) g.Members [g.Members.Count-1];
 						var f = (CodeMemberField) t.Members [t.Members.Count-1];
 						string[] values = m.Groups [1].Value.Split (new[]{','}, StringSplitOptions.RemoveEmptyEntries);

src/Xamarin.Android.Build.Tasks/Utilities/MD2Managed.cs

@@ -27,7 +27,7 @@
 // WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 //
 
-#nullable disable
+#nullable enable
 
 using System;
 
@@ -80,7 +80,7 @@ private byte[] Padding (int nLength)
 					padding[i] = (byte) nLength;
 				return padding;
 			}
-			return null;
+			throw new ArgumentException ("Length must be greater than 0", nameof (nLength));
 		}
 
 		//--- constructor -----------------------------------------------------------