Correct assert in CustomModifierUtils

The assert for custom modifier checking should include the nullable
validation

closes dotnet#30740

src/Compilers/CSharp/Portable/Symbols/Source/CustomModifierUtils.cs

@@ -102,7 +102,7 @@ internal static TypeSymbol CopyTypeCustomModifiers(TypeSymbol sourceType, TypeSy
             Debug.Assert(resultType.Equals(sourceType, TypeCompareKind.IgnoreDynamicAndTupleNames | TypeCompareKind.IgnoreNullableModifiersForReferenceTypes)); // Same custom modifiers as source type.
 
             Debug.Assert(resultType.Equals(destinationType,
-                TypeCompareKind.IgnoreCustomModifiersAndArraySizesAndLowerBounds | TypeCompareKind.IgnoreNullableModifiersForReferenceTypes)); // Same object/dynamic and tuple names as destination type.
+                TypeCompareKind.IgnoreCustomModifiersAndArraySizesAndLowerBounds)); // Same object/dynamic and tuple names as destination type.
 
             return resultType;
         }