(Bug 40860) make purgeRedundantText not fail on pre MW1.5 records

Archive records for deleted pages do not have an ar_text_id,
if they were deleted before big schema overhaul in MW 1.5.
This script was assuming all archive records had an ar_text_id.

Patch by Dévai Tamás.

Change-Id: I4776a8a0f29b8299ec6d27949dc53a96ece81f39