Ensure TypeChecker#getTypeAtLocation returns a defined result

[ghost]

Fixes #25577

[sandersn]

I guess no baselines changed because all the other callers were already correct?

In theory it would be nice to get a test case based on the bug this fixes, although with just a stack trace you'd have to make it from scratch.

[ghost]

This function was basically assumed to return a defined result everywhere. It's only had the | undefined type annotation since #22088.
I tried getting a repro but couldn't find a situation where it would return undefined and we have signature help -- may have to do with parse error handling causing an unusual node to be present there.