Move Title null check in Special:RevisionDelete

Move Title null check in Special:RevisionDelete

The property $this->targetObj was used before null check in
SpecialRevisionDelete::getList

Move up the existing null check to avoid the fatal:

Catchable fatal error: Argument 3 passed to
RevisionDeleter::createList() must be an instance of Title, null given,
called in \includes\specials\SpecialRevisiondelete.php on line 354 and
defined in \includes\revisiondelete\RevisionDeleter.php on line 83

On request with an unknown rev id

Follow-Up: Ic078af5417798db1b2900057dbf7514112e94b46
Bug: T96538
Change-Id: I0c7913ecb11ffce844add7b1f3b810d54c367519