libctf, create: member names of "" and NULL should be the same

libctf, create: member names of "" and NULL should be the same

This matters for the case of unnamed bitfields, whose names are the null
string.  These are special in that they are the only members whose
"names" are allowed to be duplicated in a single struct, but we were
only handling this for the case where name == NULL.  Translate "" to
NULL to help callers.

libctf/
	* ctf-create.c (ctf_add_member_offset): Support names of ""
	as if they were the null pointer.