Don't clobber revision size of 0

Don't clobber revision size of 0

When a revision's size is 0, don't clobber it by changing it to null. This
causes, among other things, moves of empty pages to not show a revision
size. If we get to where the else used to be, either mSize is 0 (and doing
nothing is correct), or mSize is already null, in which case this didn't
do anything anyway. We know this because, above, we initialized it with
$this->mSize = isset( $row['len'] ) ? intval( $row['len'] ) : null;

Change-Id: I3d9b9163368c57b9a7906f6569119081b945d57e