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

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

Change-Id: I89624dba3a8405cd2b8f81e64d4d83d0239a70dc