Default to empty string

Otherwise `null` is returned which leads to a ton of similar error messages if the log condition setting is enabled.:

```
{"reqId":"3wVBarnDfOV3qkI1bVCU","remoteAddr":"89.98.78.41","app":"PHP","message":"hash_equals(): Expected user_string to be a string, null given at \/var\/www\/html\/lib\/private\/Log.php#275","level":3,"time":"2016-10-07T14:22:15+00:00","method":"PROPFIND","url":"\/remote.php\/webdav\/","user":"icewind"}
```