[css-syntax-3] 'parse a list' needs to early-out on empty values, so whitespace doesn't trigger spurious failures.

Author: tabatkins

css-syntax-3/Overview.bs

@@ -2086,6 +2086,10 @@ Parse A Comma-Separated List According To A CSS Grammar</h4>
 				[=Normalize=] |input|,
 				and set |input| to the result.
 
+			<li>
+				If |input| contains only <<whitespace-token>>s,
+				return an empty [=list=].
+
 			<li>
 				<a>Parse a comma-separated list of component values</a> from |input|,
 				and let <var>list</var> be the return value.