Don't crash when receiving an RTM_DELNEIGH or NUD_FAILED.

These events don't have MAC addresses, so the code attempts to
create an Ipv6ForwardingRule with a null MAC address. This
crashes when attempting to get the raw MAC address bytes to send
to netd in the TetherOffloadRuleParcel.

This was not caught by unit tests because the test exercise this
code path in a way that is not correct (by sending RTM_DELNEIGH
and NUD_FAILED events with MAC addresses). Fix the unit tests to
properly pass in null MAC addresses for these events.

Bug: 153697068
Test: fixed existing tests to be more realistic
Change-Id: I26d89a81f1c448d9b4809652b079a5f5eace3924