No queue specified and no default queue configured #5

## Bug description

`leaf queue:work` ignores the configured default queue when the optional `--queue` option is omitted. It reports:

> No queue specified and no default queue configured. Set `default` in your queue config or pass --queue.

The application has a valid default queue configured, and `MvcConfig('queue')` returns it correctly.

## Steps to reproduce

1. Configure a default queue:

   ```php
   return [
       'default' => 'database',
       'connections' => [
           'database' => [
               'driver' => 'database',
               // ...
           ],
       ],
   ];

2. Run:

   leaf queue:work

3. The command reports that no queue is configured instead of using database.

Expected behavior

When --queue is omitted, the worker should use the default value from the queue configuration.

Actual behavior

The worker reports that no queue is configured, even though MvcConfig('queue')['default'] contains database.

Proposed solution

In QueueWorkCommand::resolveQueueConnection(), treat an empty option like an omitted option when selecting the queue:

$queue = $option ?: ($config['default'] ?? null);

This allows the configured default to be used when Sprout returns an empty string for the missing optional --queue option. Explicit queue names continue to take precedence.

Environment

• leafs/queue: v5.0
• leafs/sprout: v5
• PHP: 8.2+
• Queue driver: database