fix: flattened optional type not optional #4317
Merged
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when flattening an argument, if the type is optional, it should be a union with undefined.
the PR implements this by adding a union node for the type of the arg. if the type is required, then the union node function is given just the original type and returns it. if the type is not required, then the union node function is given the original type and the undefined keyword, creating an optional type.
see issue #4304 for inspiration