PostgreSQL: Allow rc_cur_time field to be null.

The column recentchanges.rc_cur_time is no longer populated by SQL,
so PostgreSQL must be changed to allow the NULL value for it.

In MySQL, the empty string is used as a surrogate for NULL, but that
is not allowed in PostgreSQL.

Bug: 61318
Change-Id: I6733ef11152d545382bec087d992515dcf9aec0b
(cherry picked from commit 8e4b41a50866e9503ed1498e6ac70b31c88d66ca)