Add null check for the taken callback am: 073b2d93e0

Original change: https://android-review.googlesource.com/c/platform/frameworks/opt/net/ethernet/+/1802597

Change-Id: I8fe9b5918e2af72d6edd7ae334f6062e32bbfdad
tree: 3805bada32032b4d5dbd42fa0e285a9e1ad13014
  1. service-t/
  2. tests/