Workaround blank rev_timestamps when doing consistency check.

Oldest rev_timestamp on enwiki is:

MariaDB [enwiki]> select * from revision order by rev_timestamp ASC limit 1;
+-----------+----------+----------------+-----------+----------------+----------------+-------------+---------+---------------+---------------------------------+
| rev_id    | rev_page | rev_comment_id | rev_actor | rev_timestamp  | rev_minor_edit | rev_deleted | rev_len | rev_parent_id | rev_sha1                        |
+-----------+----------+----------------+-----------+----------------+----------------+-------------+---------+---------------+---------------------------------+
| 908493298 | 26323569 |          86852 |  12509234 | 20010115192713 |              0 |           0 |      26 |             0 | hjnc5wxv75ckwvos9wsd0as31nmnice |
+-----------+----------+----------------+-----------+----------------+----------------+-------------+---------+---------------+---------------------------------+
1 row in set (0.001 sec)

Since that was the first wiki, we should be safe to default to '20000101000000'

Bug: T372677