Fix token pasting with empty fallbacks.

When deciding whether we'd need to insert a dummy comment between
two tokens in CSS token pasting, an empty fallback could confuse
the logic, in that we thought we inserted an EOF token.

Fix by having an early return from empty fallbacks; they shouldn't
affect the output in any reasonable way.

Fixed: 426549962
Change-Id: I0d1f330dd5a507c660395e0d7089873fa5382d12
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6650923
Auto-Submit: Steinar H Gunderson <[email protected]>
Commit-Queue: Anders Hartvoll Ruud <[email protected]>
Reviewed-by: Anders Hartvoll Ruud <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1477216}

Author: Steinar H. Gunderson

css/css-syntax/serialize-consecutive-tokens.html

@@ -123,4 +123,15 @@
 testComments("var(--t1)b", "\"a/* unfinished \"", "\"a/* unfinished \"b");
 testComments("var(--t1)b", "'a \" '/* comment */", "'a \" 'b");
 
+test(()=>{
+    const b = document.body;
+    b.style.setProperty("--t1", "a");
+    b.style.setProperty("--t2", "b");
+    b.style.setProperty("--result", "var(--t1)var(--does-not-exist,)var(--t2)");
+    const result = getComputedStyle(b).getPropertyValue("--result");
+    assert_equals(result[0], "a", `Result must start with a`);
+    assert_equals(result[result.length - 1], "b", `Result must end with b`);
+    assert_not_equals(result, "ab", `Result must have a comment between a and b`);
+}, 'Empty fallback between tokens must not disturb comment insertion');
+
 </script>