[obj2yaml] Produce correct output for invalid relocations.

R_X86_64_NONE can be emitted without a symbol associated (well,
in theory it should never be emitted in an ABI-compliant relocatable
object). So, if there's no symbol associated to a reloc, emit one
with an empty name, instead of crashing.

Ack'ed by Michael Spencer offline.

PR: 31768
llvm-svn: 293224