xundump: Permit omitting trailing sectors and realms

xundump: Permit omitting trailing sectors and realms

We currently require all rows to be present for tables item, sect-chr,
infrastructure, sect, realm.

The first three make sense: the code hard-codes indexes for them, and
malfunctions when entries are blank, so we want to make it hard to
leave any blank by accident.

The last two don't: blank sectors and realms work fine.  There, the
restriction is arbitrary.  Drop it.

Sectors and realms still can't be omitted "in the middle" (can do that
only with an ID selector), but that's coming soon.

See also commit 4a4ec91.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>