Updated the code from luigifab comments

Author: fballiano

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

@@ -461,14 +461,17 @@ protected function _setFilterValues($data)
     protected function _addColumnFilterToCollection($column)
     {
         if ($this->getCollection()) {
-            $field = ( $column->getFilterIndex() ) ? $column->getFilterIndex() : $column->getIndex();
+            $field = $column->getFilterIndex() ? $column->getFilterIndex() : $column->getIndex();
             if ($column->getFilterConditionCallback() && $column->getFilterConditionCallback()[0] instanceof self) {
                 call_user_func($column->getFilterConditionCallback(), $this->getCollection(), $column);
             } else {
                 $cond = $column->getFilter()->getCondition();
                 if ($field && isset($cond)) {
-                    if (in_array('NULL', array_values($cond), true)) {
-                        $this->getCollection()->addFieldToFilter($field, array('null' => true));
+                    $filtered = array_map(static function ($value) {
+                        return is_object($value) ? $value->__toString() : $value;
+                    }, array_values($cond));
+                    if (in_array('\'%NULL%\'', $filtered, true) || in_array('NULL', $filtered, true)) {
+                        $this->getCollection()->addFieldToFilter($field, ['null' => true]);
                     } else {
                         $this->getCollection()->addFieldToFilter($field, $cond);
                     }