Fixed export var default value in PackedScene when script is not loaded in editor

(cherry picked from commit 4e14eefd94a4e211fb7d908fe8d74bddd5429c6f)

scene/resources/packed_scene.cpp

@@ -481,6 +481,11 @@ Error SceneState::_parse_node(Node *p_owner, Node *p_node, int p_parent_idx, Map
 	p_node->get_property_list(&plist);
 	StringName type = p_node->get_class();
 
+	Ref<Script> script = p_node->get_script();
+	if (script.is_valid()) {
+		script->update_exports();
+	}
+
 	for (List<PropertyInfo>::Element *E = plist.front(); E; E = E->next()) {
 
 		if (!(E->get().usage & PROPERTY_USAGE_STORAGE)) {
@@ -497,7 +502,6 @@ Error SceneState::_parse_node(Node *p_owner, Node *p_node, int p_parent_idx, Map
 			isdefault = bool(Variant::evaluate(Variant::OP_EQUAL, value, default_value));
 		}
 
-		Ref<Script> script = p_node->get_script();
 		if (!isdefault && script.is_valid() && script->get_property_default_value(name, default_value)) {
 			isdefault = bool(Variant::evaluate(Variant::OP_EQUAL, value, default_value));
 		}