use `Base.promote_op` instead of `Core.Compiler.return_type`

[oscardssmith]

This moves to using a semi-documented interface from a compiler internal function (and at the same time, fixes the issues with Tuple{Union{}} on 1.10).

[aplavin]

fixes the issues with Tuple{Union{}} on 1.10

Would be useful to add a test demonstrating what's fixed here.

I think Union{} components aren't tested explicitly for now – Union{} was just a regular type that worked same as any other, before 1.10.