Bug 41801: (QA follow-up) Guard lowestPriority rank clamping against empty rank

When FixPriority is called with a reserve_id but no rank (rank defaults to ''),
the empty string evaluates to 0 numerically, satisfying `$rank <=
$highest_non_lowest_priority`. This caused the clamping block to unexpectedly
set rank to a positive value, which then triggered an unintended splice and
moved the hold.

Add `$rank > 0` to the guard condition so the block only fires when an explicit
positive rank was requested, matching the pre-existing semantics of the splice
block below it.

(cherry picked from commit 4f32ba4323)
Signed-off-by: Jacob O'Mara <Jacob.omara@openfifth.co.uk>

C4/Reserves.pm

@@ -1812,7 +1812,7 @@ sub FixPriority {
     }
 
     # if this hold is marked lowest priority, we can only move it so far
-    if ( $hold && $hold->lowestPriority && $rank ne 'del' ) {
+    if ( $hold && $hold->lowestPriority && $rank ne 'del' && $rank > 0 ) {
         my $query = "
             SELECT max(priority)
             FROM reserves