Update DrushTestTrait.php (#5076)

If `$value` is optional, we need to allow null values.

src/TestTraits/DrushTestTrait.php

@@ -90,7 +90,7 @@ public function drush($command, array $args = [], array $options = [], $site_spe
      * @param string $value The option value (or empty)
      * @return string
      */
-    protected function convertKeyValueToFlag(string $key, string $value)
+    protected function convertKeyValueToFlag(string $key, ?string $value)
     {
         if (!isset($value)) {
             return "--$key";