Set "cms.root" URL in addition to Path

[christianwach]

Overview

The code in Civi::Paths::getVariable() does not ensure all attributes of a named variable are set, so partially setting the attributes of cms.root sometimes causes "Cannot resolve path using 'cms.root.url'" exceptions to be thrown. This is relatively obscure because it requires a particular WordPress Multisite setup to occur, but the effect is repeatable.

Before

"Cannot resolve path using 'cms.root.url'" exceptions are thrown. "Settings - Resource URLs" screen renders incorrectly in WordPress Multisite on a Subsite:

After

"Cannot resolve path using 'cms.root.url'" exceptions are not thrown. "Settings - Resource URLs" screen renders correctly in WordPress Multisite on a Subsite.

Technical Details

This PR is necessary because of an incomplete check for the existence of all attributes in Paths.php. Having said that, it's still better to assign both attributes early than have Civi::Paths::getVariable() recalculate them later.

[kcristiano]

code is sensible. Should have been there from the start.

I did an r-run on master and works as expected.