Use empty-string for overwritewebroot when nextcloud_path_prefix is /

Some instances seem to have trouble when overwritewebroot is set to `/`:

> Dec 14 11:51:37 s3 systemd[1]: Starting Runs the cronjobs For Nextcloud (mash-nextcloud)...
> Dec 14 11:51:37 s3 env[1246702]: Sabre\DAV\Exception\Forbidden: Requested uri (/remote.php/dav/) is out of base uri (//remote.php/dav/) in /var/www/html/3rdparty/sabre/dav/lib/DAV/Server.php:576
> Dec 14 11:51:37 s3 env[1246702]: Stack trace:
> Dec 14 11:51:37 s3 env[1246702]: #0 /var/www/html/3rdparty/sabre/dav/lib/DAV/Server.php(542): Sabre\DAV\Server->calculateUri('/remote.php/dav...')
> Dec 14 11:51:37 s3 env[1246702]: #1 /var/www/html/apps/photos/lib/Listener/SabrePluginAuthInitListener.php(50): Sabre\DAV\Server->getRequestUri()
> Dec 14 11:51:37 s3 env[1246702]: #2 /var/www/html/lib/private/EventDispatcher/ServiceEventListener.php(86): OCA\Photos\Listener\SabrePluginAuthInitListener->handle(Object(OCA\DAV\Events\SabrePluginAuthInitEvent))
> Dec 14 11:51:37 s3 env[1246702]: #3 /var/www/html/3rdparty/symfony/event-dispatcher/EventDispatcher.php(230): OC\EventDispatcher\ServiceEventListener->__invoke(Object(OCA\DAV\Events\SabrePluginAuthInitEvent), 'OCA\\DAV\\Events\\...', Object(Symfony\Component\EventDispatcher\EventDispatcher))
> Dec 14 11:51:37 s3 env[1246702]: #4 /var/www/html/3rdparty/symfony/event-dispatcher/EventDispatcher.php(59): Symfony\Component\EventDispatcher\EventDispatcher->callListeners(Array, 'OCA\\DAV\\Events\\...', Object(OCA\DAV\Events\SabrePluginAuthInitEvent))
> Dec 14 11:51:37 s3 env[1246702]: #5 /var/www/html/lib/private/EventDispatcher/EventDispatcher.php(94): Symfony\Component\EventDispatcher\EventDispatcher->dispatch(Object(OCA\DAV\Events\SabrePluginAuthInitEvent), 'OCA\\DAV\\Events\\...')
> Dec 14 11:51:37 s3 env[1246702]: #6 /var/www/html/lib/private/EventDispatcher/EventDispatcher.php(106): OC\EventDispatcher\EventDispatcher->dispatch('OCA\\DAV\\Events\\...', Object(OCA\DAV\Events\SabrePluginAuthInitEvent))
> Dec 14 11:51:37 s3 env[1246702]: #7 /var/www/html/apps/dav/lib/CalDAV/InvitationResponse/InvitationResponseServer.php(76): OC\EventDispatcher\EventDispatcher->dispatchTyped(Object(OCA\DAV\Events\SabrePluginAuthInitEvent))
> Dec 14 11:51:37 s3 env[1246702]: #8 [internal function]: OCA\DAV\CalDAV\InvitationResponse\InvitationResponseServer->__construct(true

It seems like changing overwritewebroot to an empty string solves the
problem there, and doesn't seem to be introducign regressions for
installations that didn't have the problem in the first place.

defaults/main.yml

@@ -324,7 +324,7 @@ nextcloud_config_default_parameters:
     value: "{{ nextcloud_url }}"
     type: string
   - key: overwritewebroot
-    value: "{{ nextcloud_path_prefix }}"
+    value: "{{ '' if nextcloud_path_prefix == '/' else nextcloud_path_prefix }}"
     type: string
   - key: htaccess.RewriteBase
     value: "{{ nextcloud_path_prefix }}"