[css-values-5] I a whole word

Author: tabatkins

css-values-5/Overview.bs

@@ -1822,7 +1822,7 @@ Substitution</h4>
 
 			1. If |second arg| is null,
 				and |syntax| was omitted,
-				return an CSS <<string>>.
+				return an empty CSS <<string>>.
 
 			2. If |second arg| is null,
 				return the [=guaranteed-invalid value=].