style: Fix container orientation evaluation

Allow keyword evaluators to return unknown.

Do you know of a good test to extend here? Otherwise I can add a new
one, though I gotta run atm.

Differential Revision: https://phabricator.services.mozilla.com/D165630

components/style/queries/feature_expression.rs

@@ -637,7 +637,7 @@ impl QueryFeatureExpression {
                     .kind
                     .non_ranged_value()
                     .map(|v| *expect!(Enumerated, v));
-                evaluator(context, computed)
+                return evaluator(context, computed)
             },
             Evaluator::BoolInteger(eval) => {
                 let computed = self