Add check for RequiredTrait

[rangerthegood]

Issue #, if available:
Resolves: #329

Description of changes:
Add an additional check for the trait RequiredTrait when making the determination if a field should be optional or not.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[JordonPhillips]

The NullableIndex already checks this. But there's also a number of special cases where other traits take priority, which is why that knowledge index exists in the first place.

[rangerthegood]

The code index.isMemberNullable(member) uses the CheckMode.CLIENT by default. I was thinking of using the generated models in a server. If I change the code to index.isMemberNullable(member, CheckMode.SERVER) this would also fix the issue I'm facing in #329.