Bug 1886377 - Fix substring attribute selectors for Servo. r=emilio

Substring attribute selectors should never match when an empty string is
provided. This fixes [attr^=""], [attr$=""], [attr*=""] and [attr~=""]
in Servo, which broke when D180529 removed never_matches.

Gecko doesn't use this code at all, so no change in behavior.

Differential Revision: https://phabricator.services.mozilla.com/D205187

servo/components/selectors/attr.rs

@@ -111,16 +111,21 @@ impl AttrSelectorOperator {
         let case = case_sensitivity;
         match self {
             AttrSelectorOperator::Equal => case.eq(e, s),
-            AttrSelectorOperator::Prefix => e.len() >= s.len() && case.eq(&e[..s.len()], s),
+            AttrSelectorOperator::Prefix => {
+                !s.is_empty() && e.len() >= s.len() && case.eq(&e[..s.len()], s)
+            },
             AttrSelectorOperator::Suffix => {
-                e.len() >= s.len() && case.eq(&e[(e.len() - s.len())..], s)
+                !s.is_empty() && e.len() >= s.len() && case.eq(&e[(e.len() - s.len())..], s)
             },
             AttrSelectorOperator::Substring => {
-                case.contains(element_attr_value, attr_selector_value)
+                !s.is_empty() && case.contains(element_attr_value, attr_selector_value)
+            },
+            AttrSelectorOperator::Includes => {
+                !s.is_empty() &&
+                    element_attr_value
+                        .split(SELECTOR_WHITESPACE)
+                        .any(|part| case.eq(part.as_bytes(), s))
             },
-            AttrSelectorOperator::Includes => element_attr_value
-                .split(SELECTOR_WHITESPACE)
-                .any(|part| case.eq(part.as_bytes(), s)),
             AttrSelectorOperator::DashMatch => {
                 case.eq(e, s) || (e.get(s.len()) == Some(&b'-') && case.eq(&e[..s.len()], s))
             },