Improve Coalesce Function for Proper Type Inference

[jongleb]

This pull request enhances the coalesce function to provide accurate type inference. Previously, coalesce didn't behave as expected, but with this update, it correctly outputs a not null type if any of the parameters are not null.

[Khady]

nitpick, but @jongleb I think you can get commit messages that are a bit more descriptive than just "coalesce"