Fix Stylist::lazily_compute_pseudo_element_style to return None when it should.

We have a fast path to return None if we have no rules for the pseudo-element at
all, but we should also return None if we have no _matching_ rules for it for
the given originating element.  This is relied on by consumers like
Servo_ResolvePseudoStyle, which needs to be able to detect the "no styles for
this pseudo-element for this originating element case", to support probing for
whether work for a specific pseudo-element should be done at all.

components/style/stylist.rs

@@ -534,6 +534,10 @@ impl Stylist {
                                           &mut declarations,
                                           &mut set_selector_flags);
 
+        if declarations.is_empty() {
+            return None
+        }
+
         let rule_node =
             self.rule_tree.insert_ordered_rules(
                 declarations.into_iter().map(|a| (a.source, a.level)));