Merge "Handle empty / undefined slice-type product variables uniformly." am:...

Merge "Handle empty / undefined slice-type product variables uniformly." am: 8f91f7d8 am: 78498eb9

Original change: https://android-review.googlesource.com/c/platform/build/soong/+/1359443

Change-Id: I786c26f0faccd6195637708c4380f7eceb042657