Fix two nullability warnings

Author: Sergio0694

components/DependencyPropertyGenerator/CommunityToolkit.DependencyPropertyGenerator.SourceGenerators/EmbeddedResources/GeneratedDependencyPropertyAttribute.cs

@@ -74,7 +74,7 @@ internal sealed class GeneratedDependencyPropertyAttribute : global::System.Attr
 #if NET8_0_OR_GREATER
         [global::System.Diagnostics.CodeAnalysis.DisallowNull]
 #endif
-        public string? DefaultValueCallback { get; init; } = null;
+        public string? DefaultValueCallback { get; init; } = null!;
 
         /// <summary>
         /// Gets a value indicating whether or not property values should be cached locally, to improve performance.

components/DependencyPropertyGenerator/src/GeneratedDependencyPropertyAttribute.cs

@@ -68,7 +68,7 @@ public sealed class GeneratedDependencyPropertyAttribute : Attribute
 #if NET8_0_OR_GREATER
     [DisallowNull]
 #endif
-    public string? DefaultValueCallback { get; init; } = null;
+    public string? DefaultValueCallback { get; init; } = null!;
 
     /// <summary>
     /// Gets a value indicating whether or not property values should be cached locally, to improve performance.