Fix "Uninitialized string offset: 0 at \/media\/psf\/stable9\/lib\/private\/URLGenerator.php#224"

The URLGenerator doesn't support `` as target for absolute URLs, we need to link to `/` thus.

Regression introduced with https://github.com/nextcloud/server/commit/46229a00f39e507249dbe3ceb7507277da3fa4f8



Signed-off-by: Lukas Reschke <lukas@statuscode.ch>