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.

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[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.