Merge "Prevent NPEs when registering/unregistering ConnDiags CBs." am: 5805d3cfd2 am: 5c5ff18089 am: 873d8c9fc6

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/1761210

Change-Id: Ia0bf3475023cf5bf928e7b8e9a6efd48c9707394