[BUG] [Dart] Nullable values are still in required list, and fail on assertion in the serializer

[isaac-merkos]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?

Description

I am expecting nullable fields to allow values, my spec allows for nullable values in certain fields but when I actually run the queries I am getting an assertion error

[uchijo]

Hi, I'm facing exactly same issue.
After seeing the generated code, I figured out that it might be caused by required but nullable value as you mention.
The tool generates code like this in fromJson method:

      assert(() {
        requiredKeys.forEach((key) {
          assert(json.containsKey(key), 'Required key "SomeModel[$key]" is missing from JSON.');
          assert(json[key] != null, 'Required key "SomeModel[$key]" has a null value in JSON.'); // this code is not good. it ignores exisitence of required but nullable value.
        });
        return true;
      }());

I will look into generator code and see if I can fix it.