lib: return "" rather than NULL from notmuch_thread_get_authors

lib: return "" rather than NULL from notmuch_thread_get_authors

The current behaviour is at best under-documented. The modified test in
T470-missing-headers.sh previously relied on printf doing the right
thing with NULL, which seems icky.

The use of talloc_strdup here is probably overkill, but it avoids
having to enforce that thread->authors is never mutated outside
_resolve_thread_authors_string.