ISSUE-1131: Added 2nd review improvements

Author: elize-vdr

src/Microsoft.OpenApi.Readers/OpenApiDiagnostic.cs

@@ -31,8 +31,8 @@ public class OpenApiDiagnostic : IDiagnostic
         /// Append another set of diagnostic Errors and Warnings to this one, this may be appended from another external
         /// document's parsing and we want to indicate which file it originated from.
         /// </summary>
-        /// <param name="diagnosticToAdd"></param>
-        /// <param name="fileNameToAdd"></param>
+        /// <param name="diagnosticToAdd">The diagnostic instance of which the errors and warnings are to be appended to this diagnostic's</param>
+        /// <param name="fileNameToAdd">The originating file of the diagnostic to be appended, this is prefixed to each error and warning to indicate the originating file</param>
         public void AppendDiagnostic(OpenApiDiagnostic diagnosticToAdd, string fileNameToAdd = null)
         {
             var fileNameIsSupplied = !string.IsNullOrEmpty(fileNameToAdd);
@@ -53,16 +53,18 @@ public void AppendDiagnostic(OpenApiDiagnostic diagnosticToAdd, string fileNameT
 /// <summary>
 /// Extension class for IList to add the Method "AddRange" used above
 /// </summary>
-public static class IDiagnosticExtensions
+internal static class IDiagnosticExtensions
 {
     /// <summary>
     /// Extension method for IList so that another list can be added to the current list.
     /// </summary>
     /// <param name="collection"></param>
     /// <param name="enumerable"></param>
     /// <typeparam name="T"></typeparam>
-    public static void AddRange<T>(this ICollection<T> collection, IEnumerable<T> enumerable)
+    internal static void AddRange<T>(this ICollection<T> collection, IEnumerable<T> enumerable)
     {
+        if (collection is null || enumerable is null) return;
+
         foreach (var cur in enumerable)
         {
             collection.Add(cur);