fix(documentator): null output in generated help of `lara-asp-docum…

…entator:preprocess` command.

packages/documentator/src/Commands/Preprocess.php

@@ -279,6 +279,7 @@ protected function getProcessedHelpTaskPreprocessParameters(
                 $default = match (true) {
                     $theDefault instanceof UnitEnum => $theDefault::class.'::'.$theDefault->name,
                     is_scalar($theDefault)          => var_export($theDefault, true),
+                    $theDefault === null            => 'null',
                     default                         => '<'.gettype($theDefault).'>',
                 };
                 $definition = "{$definition} = `{$default}`";