[DWARF][FIX] Try not to crash for nvptx with missing debug information

This prevents crashes in the OpenMP offload pipeline as not everything
is properly annotated with debug information, e.g., the runtimes we link
in. While we might want to have them annotated, it seems to be generally
useful to gracefully handle missing debug info rather than crashing.

TODO: A test is missing and can hopefully be distilled prior to landing.

This fixes #51079.

Differential Revision: https://reviews.llvm.org/D116959