Fix JsonValueReaderWriter check to support polymorphism (npgsql#3339)

Fixes npgsql#3169

Author: roji

src/EFCore.PG/Storage/Internal/Mapping/NpgsqlArrayTypeMapping.cs

@@ -142,10 +142,10 @@ private static RelationalTypeMappingParameters CreateParameters(string storeType
 #pragma warning restore EF1001
 
         var elementJsonReaderWriter = elementMapping.JsonValueReaderWriter;
-        if (elementJsonReaderWriter is not null && elementJsonReaderWriter.ValueType != typeof(TElement).UnwrapNullableType())
+        if (elementJsonReaderWriter is not null && !typeof(TElement).UnwrapNullableType().IsAssignableTo(elementJsonReaderWriter.ValueType))
         {
             throw new InvalidOperationException(
-                $"When building an array mapping over '{typeof(TElement).Name}', the JsonValueReaderWriter for element mapping '{elementMapping.GetType().Name}' is incorrect ('{elementMapping.JsonValueReaderWriter?.GetType().Name ?? "<null>"}' instead of '{typeof(TElement).UnwrapNullableType()}').");
+                $"When building an array mapping over '{typeof(TElement).Name}', the JsonValueReaderWriter for element mapping '{elementMapping.GetType().Name}' is incorrect ('{elementJsonReaderWriter.ValueType.GetType().Name}' instead of '{typeof(TElement).UnwrapNullableType()}', the JsonValueReaderWriter is '{elementJsonReaderWriter.GetType().Name}').");
         }
 
         // If there's no JsonValueReaderWriter on the element, we also don't set one on its array (this is for rare edge cases such as