pref-filters: fix malformed JSON when there are no filters defined

modules/pref-filters.php

@@ -35,6 +35,8 @@
 			$cat = false;
 			$cur_action_description = "";
 
+			if (db_num_rows($result) > 0) {
+
 				while ($line = db_fetch_assoc($result)) {
 					if ($cur_action_description != $line['action_description']) {
 	
@@ -89,6 +91,7 @@
 				}
 	
 				array_push($root['items'], $cat);
+			}
 
 			$fl = array();
 			$fl['identifier'] = 'id';