pg_dump: store unused attribs as NULL instead of '\0'

pg_dump: store unused attribs as NULL instead of '\0'

Commit f831d4accda0 changed pg_dump to emit (and pg_restore to
understand) NULLs for unused members in ArchiveEntry structs, as a side
effect of some code beautification.  That broke pg_restore of dumps
generated with older pg_dump, however, so it was reverted in
19455c9f5606.  Since the archiver version number has been bumped in
3b925e905de3, we can put it back.

Author: Dmitry Dolgov
Discussion: https://postgr.es/m/CA+q6zcXx0XHqLsFJLaUU2j5BDiBAHig=YRoBC_YVq7VJGvzBEA@mail.gmail.com