ModerationCenter.controller: check if $watched_users isn't empty
ModerationCenter.controller: check if $watched_users isn't empty

Fixes that you can't visit the moderation center when $watched_users is empty.

```
PHP Fatal error: Uncaught exception 'ErrorException' with message 'Invalid argument supplied for foreach()' in /home/frans/www/public_html/elkarte-1.1/sources/controllers/ModerationCenter.controller.php:1710
Stack trace:
thrown in /home/frans/www/public_html/elkarte-1.1/sources/controllers/ModerationCenter.controller.php on line 1710
```

emanuele45/Dialogo,elkarte/Elkarte,elkarte/Elkarte,emanuele45/Dialogo,interlab/Elkarte,interlab/Elkarte,Spuds/Elkarte,interlab/Elkarte,StealthWombat/How-to-fork-an-Elk,Spuds/Elkarte,StealthWombat/How-to-fork-an-Elk,elkarte/Elkarte,nwsw/Elkarte,StealthWombat/How-to-fork-an-Elk,Spuds/Elkarte,nwsw/Elkarte,nwsw/Elkarte,StealthWombat/How-to-fork-an-Elk,emanuele45/Dialogo