[DX] Fix run on custom rector config when no rector.php and no rules/sets registered

[samsonasik]

Fixes rectorphp/rector#9757

When no rules/sets registered in custom rector config.

Before

  rector-src git:(main) ✗ bin/rector --config="custom-rector.php" --dry-run

 No "rector.php" config found. Should we generate it for you? [yes]:
 > no

After

bin/rector --config="custom-rector.php" --dry-run 
                                                                                                                        
 [WARNING] Register rules or sets in your "custom-rector.php" config     

Steps to reproduce:

• no rector.php
• create custom-rector.php without rule/sets registered
• run rector with --config="custom-rector.php"

[samsonasik]

Fixed 🎉 /cc @Art4

[samsonasik]

@TomasVotruba ready 👍