LibJS: Make Value() default-construct the undefined value

The special empty value (that we use for array holes, Optional<Value>
when empty and a few other other placeholder/sentinel tasks) still
exists, but you now create one via JS::js_special_empty_value() and
check for it with Value::is_special_empty_value().

The main idea here is to make it very unlikely to accidentally create an
unexpected special empty value.

Libraries/LibJS/Runtime/VM.h

@@ -175,7 +175,7 @@ public:
 
     Value this_value() const
     {
-        return running_execution_context().this_value;
+        return running_execution_context().this_value.value();
     }
 
     ThrowCompletionOr<Value> resolve_this_binding();