Don't try to store null values in the DB for variables; closes pterodactyl#3038

Author: DaneEveritt

app/Services/Servers/ServerCreationService.php

@@ -258,7 +258,7 @@ private function storeEggVariables(Server $server, Collection $variables)
             return [
                 'server_id' => $server->id,
                 'variable_id' => $result->id,
-                'variable_value' => $result->value,
+                'variable_value' => $result->value ?? '',
             ];
         })->toArray();