[RTT] Add NPE checks for invalid MAC addresses from Aware

RTT requests for Aware peers may use PeerHandles. The RTT service
queries the Aware service for their corresponding MAC addresses. In
some corner cases, e.g. the Aware peer just disappeared, a null MAC
address may be returned.

This was not handled, resulting in an NPE.

Bug: 111646773
Test: new unit test added, passes
Change-Id: I7d7c37322848fe7e7dd4f9f412f1e1339321e9a8
2 files changed