Add SUCCESS return

To fix;

```
  [TypeError]                                                                                                                                               
  Return value of "Vaimo\ComposerPatches\Composer\Commands\ListCommand::execute()" must be of the type int, "null" returned.
```

Author: peterjaap

src/Composer/Commands/ListCommand.php

@@ -194,6 +194,8 @@ protected function execute(InputInterface $input, OutputInterface $output)
         }
 
         $this->generateOutput($output, $patches);
+        
+        return self::SUCCESS;
     }
 
     private function createUnfilteredPatchLoaderPool(\Vaimo\ComposerPatches\Composer\Context $composerContext)