Make populateRevisionLength fix rows with ar_len = 0.

Due to T192189, some rows in the archive table may have ar_len set to 0
erronously. This can be detected by checking if they have ar_sha1 set to
"phoiac9h4m842xq45sp7s6u21eteeq1", which is the hash of the empty string.

This patch makes populateRevisionLength.php detect and fix such rows.

Bug: T192189
Change-Id: I9b6cd62f4b8c5bf93cd305b56af4352a527e19c9
(cherry picked from commit e420790f5200568422dce9473ef44d4f7cefe675)