[Templight] Don't display empty strings for names of unnamed template parameters

Patch originally by oktal3000: https://github.com/mikael-s-persson/templight/pull/40

When a template parameter is unnamed, the name of -templight-dump might return
an empty string. This is fine, they are unnamed after all, but it might be more
user friendly to at least describe what entity is unnamed.

Differential Revision: https://reviews.llvm.org/D115521