[typescript-fetch] Fix for nullable enums + separate treatment of null and undefined

Created by: jtbeach

The typescript-fetch generator can still generate code with type failures in the case of nullable enum values. This addresses this by applying a similar fix that was made to modelGenericMustache to modelEnumMustache.

In addition, tweak the previous fix to modelGenericMustache so that it handles null and undefined separately rather than using == to map them both to undefined. There is a difference between a key existing vs. a key existing with a null value.

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