[DexTer] Add step.UNKNOWN check for NoneType line numbers.

Summary: It's possible for an instance of the visual studio debugger
to return a NoneType line number location when stepping during a
debugging session.

This patches teaches DexTer how to handle this particular case without
crashing out.

Reviewers: Orlando

Differential revision: https://reviews.llvm.org/D75992

GitOrigin-RevId: 09f4bdc03f891cbcf0c5633c8c7013eb0fbe626f
1 file changed