Allow admin to enter a bonus score of zero

Closes #87

app/Http/Controllers/Tabulation/BonusScoreController.php

@@ -69,7 +69,7 @@ class BonusScoreController extends Controller
             }
 
             // If no score was submitted, were going to just stop at deleting the scores
-            if (! $validData['score'] == null) {
+            if (! is_null($validData['score'])) {
                 // Set the new score
                 try {
                     $saveBonusScore($judge, $entry, $validData['score']);