Refactor Union conversion.
Refactor Union conversion.

Do None to NoneType conversion only once when getting types. It seems
`typing.get_type_hints` does that for us but we still need to handle
cases like `@keyword` where types are given directly.

robotframework/robotframework,HelioGuilherme66/robotframework,HelioGuilherme66/robotframework,HelioGuilherme66/robotframework,robotframework/robotframework,robotframework/robotframework