Refine ComponentClass::class_empty

ComponentClass::class_empty should only be used on finalized classes to
determine whether a given class contains any elements; it should not
take the cr_ucp or cps_ucp into account, as they have been folden in by
the finalize call.

Fixes our failure to identify that the pattern /[^\D\d]/8W can never
match.

src/parser/AsciiComponentClass.cpp

@@ -52,7 +52,8 @@ AsciiComponentClass *AsciiComponentClass::clone() const {
 }
 
 bool AsciiComponentClass::class_empty(void) const {
-    return cr.none() && cr_ucp.none();
+    assert(finalized);
+    return cr.none();
 }
 
 void AsciiComponentClass::createRange(unichar to) {