[llvm-nm] Add test for `--debug-syms --dynamic`

Summary: This test ensures that `llvm-nm` will omit NULL symbol.

Reviewers: jhenderson, MaskRay, grimar

Reviewed By: jhenderson, grimar

Subscribers: rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76213