Fix #127976: Actions created via Python can't be used in action constraints

The Action Constraint would only allow actions that were once assigned
to objects, i.e. with `action->idroot == ID_OB`. This did not take into
account the equally valid `0` case, which is what indicates that this
Action has never been assigned to anything yet.

Pull Request: https://projects.blender.org/blender/blender/pulls/128072

source/blender/editors/object/object_constraint.cc

@@ -360,7 +360,7 @@ static void test_constraint(
     else {
       animrig::Action &action = data->act->wrap();
       if (action.is_action_legacy()) {
-        if (data->act->idroot != ID_OB) {
+        if (data->act->idroot != ID_OB && data->act->idroot != 0) {
           /* Only object-rooted actions can be used. */
           data->act = nullptr;
           con->flag |= CONSTRAINT_DISABLE;