Fixed admin grid filter (#2537)

E.g. customer review: filter by customer group

Author: sreichel

app/code/core/Mage/Adminhtml/Block/Widget/Grid.php

@@ -500,7 +500,7 @@ protected function _addColumnFilterToCollection($column)
                 if ($field && $cond !== null) {
                     $filtered = array_map(static function ($value) {
                         return is_object($value) ? $value->__toString() : $value;
-                    }, array_values($cond));
+                    }, is_array($cond) ? array_values($cond) : [$cond]);
                     if (in_array('\'%NULL%\'', $filtered, true) || in_array('NULL', $filtered, true)) {
                         $this->getCollection()->addFieldToFilter($field, ['null' => true]);
                     } else {