Fixed error causing permissions to be deleted on book sort

Closes #282

@@ -208,7 +208,7 @@ class BookController extends Controller
         }
 
         // Update permissions on changed models
-        $this->entityRepo->buildJointPermissions($updatedModels);
+        if (count($updatedModels) === 0) $this->entityRepo->buildJointPermissions($updatedModels);
 
         return redirect($book->getUrl());
     }

@@ -243,6 +243,7 @@ class PermissionService
      */
     protected function deleteManyJointPermissionsForEntities($entities)
     {
+        if (count($entities) === 0) return;
         $query = $this->jointPermission->newQuery();
             foreach ($entities as $entity) {
                 $query->orWhere(function($query) use ($entity) {