Implement the possibility to search for null in backend grids

.github/phpstan-baseline.neon

@@ -747,7 +747,7 @@ parameters:
 
 		-
 			message: "#^Call to an undefined method Varien_Data_Collection\\:\\:addFieldToFilter\\(\\)\\.$#"
-			count: 1
+			count: 2
 			path: ../app/code/core/Mage/Adminhtml/Block/Widget/Grid.php
 
 		-

README.md

@@ -98,8 +98,9 @@ Most important changes will be listed here, all other changes since `19.4.0` can
 
 ### Between Magento 1.9.4.5 and OpenMage 19.x
 
-- bug fixes and PHP 7.x and 8.0 compatibility
+- bug fixes and PHP 7.x, 8.0 and 8.1 compatibility
 - added config cache for system.xml [#1916](https://github.com/OpenMage/magento-lts/pull/1916)
+- search for "NULL" in backend grids [#1203](https://github.com/OpenMage/magento-lts/pull/1203)
 
 ### Between OpenMage 19.x and 20.x
 

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

@@ -496,7 +496,14 @@ protected function _addColumnFilterToCollection($column)
             } else {
                 $cond = $column->getFilter()->getCondition();
                 if ($field && $cond !== null) {
-                    $this->getCollection()->addFieldToFilter($field , $cond);
+                    $filtered = array_map(static function ($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);
+                    }
                 }
             }
         }