Fix #21669: Check parents non-empty before calling reduceLeft

[noti0na1]

Fix #21669

This part of code is from #20084

We should check parents is non-empty before calling reduceLeft.

I haven't been able to create a standalone test.

[noti0na1]

Hi @odersky @EugeneFlesselle , I saw you worked on #20084. Do you think an empty parents is a valid case?

We also have many other places where we call parents.reduceLeft directly, do you think we need to concern these places as well? I guess these places are ok because the types must be classes/enums.