Strlen callback function

This PR is similar to #4083. It replaces the callback function '\strlen' with an arrow function (introduced in PHP 7.4) to avoid the error message bellow starting with PHP >8.1

Deprecated functionality: strlen(): Passing null to parameter #1

app/code/core/Mage/ImportExport/Model/Import/Entity/Product.php

@@ -1972,7 +1972,7 @@ protected function _saveStockItem()
      */
     protected function _filterRowData(&$rowData)
     {
-        $rowData = array_filter($rowData, '\strlen');
+        $rowData = array_filter($rowData, fn ($tmpString) => strlen($tmpString ?? ''));
         // Exceptions - for sku - put them back in
         if (!isset($rowData[self::COL_SKU])) {
             $rowData[self::COL_SKU] = null;