Reject empty pg_hba.conf files.

Reject empty pg_hba.conf files.

An empty HBA file is surely an error, since it means there is no way to
connect to the server.  We've not heard identifiable reports of people
actually doing that, but this will also close off the case Thom Brown just
complained of, namely pointing hba_file at a directory.  (On at least some
platforms with some directories, it will read as an empty file.)

Perhaps this should be back-patched, but given the lack of previous
complaints, I won't add extra work for the translators.