[llvm-gsymutil] Fix assert failure on FileEntry.Dir empty (#79926)

Summary:
FileEntry.Dir can be empty if debug info only contains relative path.
This caused an assertion failure when gsym segmentation is trying to
copy a file entry with empty dir. As the fitst entry of StringTable is
always empty (and is preserved), `StringOffsetMap` doesn't have key 0.
Hence, `find(0)` returns `End` and `operator->()` fails the assertion

Test Plan:
./bin/llvm-lit -sv llvm/test/tools/llvm-gsymutil/X86/elf-empty-dir.yaml