Fixed an issue with base type list formatting

Microsoft.Toolkit.Mvvm.SourceGenerators/TransitiveMembersGenerator.cs

@@ -81,7 +81,7 @@ private void OnExecute(
         {
             ClassDeclarationSyntax sourceDeclaration = sourceSyntaxTree.GetRoot().DescendantNodes().OfType<ClassDeclarationSyntax>().First();
             UsingDirectiveSyntax[] usingDirectives = sourceSyntaxTree.GetRoot().DescendantNodes().OfType<UsingDirectiveSyntax>().ToArray();
-            BaseListSyntax baseListSyntax = BaseList(SeparatedList(
+            BaseListSyntax? baseListSyntax = BaseList(SeparatedList(
                 sourceDeclaration.BaseList?.Types
                 .OfType<SimpleBaseTypeSyntax>()
                 .Select(static t => t.Type)
@@ -91,6 +91,11 @@ private void OnExecute(
                 .ToArray()
                 ?? Array.Empty<BaseTypeSyntax>()));
 
+            if (baseListSyntax.Types.Count == 0)
+            {
+                baseListSyntax = null;
+            }
+
             // Create the class declaration for the user type. This will produce a tree as follows:
             //
             // <MODIFIERS> <CLASS_NAME> : <BASE_TYPES>

Microsoft.Toolkit.Mvvm/ComponentModel/Attributes/ObservableRecipientAttribute.cs

@@ -26,6 +26,10 @@ namespace Microsoft.Toolkit.Mvvm.ComponentModel
     /// </code>
     /// </para>
     /// And with this, the same APIs from <see cref="ObservableRecipient"/> will be available on this type as well.
+    /// <para>
+    /// To avoid conflicts with other APIs in types where the new members are being generated, constructors are omitted. Make sure to
+    /// properly initialize the <see cref="ObservableRecipient.Messenger"/> property from the constructors in the type being annotated.
+    /// </para>
     /// </summary>
     /// <remarks>
     /// In order to work, <see cref="ObservableRecipientAttribute"/> needs to be applied to a type that inherits from