Request: fix handling of empty overwritewebroot.

There are situations in which I believe an empty string value for ```overwritewebroot``` is required, such as when handling requests passed from the web root ```/``` to an installation in a subdirectory ```/owncloud```, as is normal on an RPM installation on CentOS with nginx acting as SSL termination and a reverse proxy.

Unfortunately, such cases are presently impossible as this value is discarded or taken to mean 'not set'. For this reason, accept ```OC_Config```'s default value of ```null``` when checking whether or not an ```overwritewebroot``` value has been configured.

Note: not particularly well tested; I'm not particularly familiar with ownCloud core either. Directions welcome.