Fixed null parameter warning in Mage_ImportExport_Model_Import_Entity_Product::_filterRowData()

[addison74]

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

Initially there were 4 variants, but considering the opinions from PR #4083 I chose the same implementation.

array_filter($VARIABLE, fn ($tmpString) => strlen($tmpString ?? ''))
// $qtys = array_filter($VARIABLE, function ($tmpString) { return strlen($tmpString ?? ''); });
// $qtys = array_filter($VARIABLE, function ($tmpString) { return $tmpString !== NULL && strlen($tmpString); }); => From Drupal Community
// $qtys = @array_filter($VARIABLE, '\strlen');