Avoid filtering out un-representable types in return annotation

[charliermarsh]

Summary

Given Union[Dict, None] (in our internal representation), we were filtering out Dict since we treat it as un-representable (i.e., we can't convert it to an expression), returning just None as the type annotation. We should require that all members of the union are representable.

Closes #8879.

[github-actions]

ruff-ecosystem results

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