[lld-macho] Avoid running LTO pipeline for no files

If no bitcode files are given as inputs but -object_path_lto is passed
we should avoid running the LTO pipeline.

This mirrors this patch from the ELF port https://github.com/llvm/llvm-project/commit/f3994e4dfa0214b2a09a0e327ba37e6b38bbcdb3

Fixes https://github.com/llvm/llvm-project/issues/61031

Differential Revision: https://reviews.llvm.org/D145273