Import: Fix php warning when importing with variable name

[Krinkle]

The import worked fine, but it produced a warning as side-effect.

Prevent this by recognising that some URL nodes assigned to $path during the first phase have $value set to a Variable node (instead of an Anonymous node), and thus don't have a value property themselves yet.

Fixes https://github.com/wikimedia/less.php/issues/29. Fixes https://github.com/wikimedia/less.php/issues/32.

[Krinkle]

Test plan suggestion:

• Run test locally without source patch to reproduce the PHP notice.
• Add var_dump(this->path) to observe that when you run bin/lessc a.less (with a.less and b.less from https://github.com/wikimedia/less.php/issues/29#issuecomment-895706701 in the git project directory), you will find the code is run multiple times. The first time, the Tee_Import node has a Tree_URL path with a Tree_Variable value (and no value property), and then later it gets run with a Tree_Anonymous value which does have a value property.