When an $ORIGIN is defined that is different from the zone name, then records with "empty names" (which refer to the previous record) are not correctly parsed.

Example input for zone "mydomain.biz.":

$ORIGIN _subdomain.mydomain.biz.

test A 192.168.2.42
     A 192.168.2.43

The second entry is parsed as "._subdomain.mydomain.biz." instead of "test._subdomain.mydomain.biz.".

I think the best fix is to check for empty name in appendOrigin(), so it is correctly handled by populateNullValues() later (as it is already handled if we are in main zone context).

This PR implements the fix with the corresponding test.