Do not generate file if there are no changes

The following code in DiffCommand.php is never run because of the "Migration can only be executed safely on XXX"-warning that is always added:

```
        if ( ! $up && ! $down) {                                                                                        
            $output->writeln('No changes detected in your mapping information.', 'ERROR');

            return;                                                                                                     
        }                                                                                                               
```

This regression was introduced in February 2011: 14f6389c99e3a4c60085ce27fe608317db962e13

With this patch, the warning is only added when there are actual changes to migrate.