various fixes, also for postgres, WIP

Fix for postgres. 

WIP, currently debugging 

```
{"app":"hook","message":"error while running hook (OCA\\Music\\Utility\\HookHandler::fileUpdated): An exception occurred while executing 'INSERT INTO \"oc_music_tracks\"(\"title\",\"number\",\"artist_id\",\"album_id\",\"file_id\",\"mimetype\",\"user_id\") VALUES(?,?,?,?,?,?,?)':\n\nSQLSTATE[22P02]: Invalid text representation: 7 ERROR:  invalid input syntax for integer: \"\"","level":3,"time":"2013-09-09T22:02:14+00:00"}
```

Which happens when number is empty