pg_dump: Fix ArchiveEntry handling of some empty values

pg_dump: Fix ArchiveEntry handling of some empty values

Commit f831d4acc changed what pg_dump emits for some empty fields: they
were output as empty strings before, NULL pointer afterwards.  That
makes old pg_restore unable to work (crash) with such files, which is
unacceptable.  Return to the original representation by explicitly
setting those struct members to "" where needed; remove some no longer
needed checks for NULL input.

We can declutter the code a little by returning to NULLs when we next
update the archive version, so add a note to remind us later.

Discussion: https://postgr.es/m/20190225074539.az6j3u464cvsoxh6@depesz.com
Reported-by: hubert depesz lubaczewski
Author: Dmitry Dolgov