[7.x] allow to reset forced scheme and root-url

[Gummibeer]

Right now it's not possible to clear/reset a forced scheme or root-url.
This is one way to create links to different URLs - for example in emails a link to a dedicated frontend.
https://twitter.com/m1guelpf/status/1283175178961387520

url()->to('/'); // http://localhost:8000

url()->forceScheme('https');
url()->forceRootUrl('https://frontend.local');

url()->to('/'); // https://frontend.local

url()->forceScheme(null);
url()->forceRootUrl(null);

url()->to('/'); // ://localhost:8000

I would expect the last one to be equal to the first one - instead I get an URL without any scheme.

This PR fixes this by setting null to the property if the forced value is empty/null/false.