Well, the only reason that static member is called var::null and not var::void is because void is a keyword. Void is a better name for that concept.
And a top tip is to avoid using var::null and prefer to just use var() if you need a void one. Same with String::empty, ValueTree::invalid, etc. It's much better for the compiler if you don't create an unnecessary reference to a static member variable that it has to assume could be volatile.
The context im using it in is checking that a property exists after parsing some json. Are you saying its better to check against an empty var() or that i could in fact use isVoid()?