Fix warnings when personal is not set

Fixes log pollution with:
```
{"reqId":"53b29c26104f8","app":"PHP","message":"Undefined index: personal at \/srv\/www\/htdocs\/owncloud\/apps\/files_external\/lib\/config.php#114","level":0,"time":"2014-07-01T11:31:50+00:00","method":"GET","url":"\/owncloud\/index.php\/apps\/files\/ajax\/getstoragestats.php?dir=%2F"}
```

Please review @icewind1991 @Xenopathic @schiesbn