Minor: Improve error message when no Queue config key present in configuration

[steinkel]

When no Queue key is present in the configuration, we get a

warning: 2 :: foreach() argument must be of type array|object, null given on line 53 of /application/vendor/cakephp/queue/src/Plugin.php
Stack Trace:

Cake\Queue\Plugin::bootstrap() /application/vendor/cakephp/queue/src/Plugin.php, line 53
Cake\Http\BaseApplication::pluginBootstrap() /application/vendor/cakephp/cakephp/src/Http/BaseApplication.php, line 182
Cake\Console\CommandRunner::bootstrap() /application/vendor/cakephp/cakephp/src/Console/CommandRunner.php, line 197
Cake\Console\CommandRunner::run() /application/vendor/cakephp/cakephp/src/Console/CommandRunner.php, line 133
[main] /application/bin/cake.php, line 12

We could display instead an error message like "Missing Queue configuration key, please check the CakePHP Queue documentation to complete the plugin setup"