Guard against missing mac addresses in onWakeupEvent

The native code may not send any data for the NFULA_HWADDR on
some networks which can result in an exception. Logging an exception if
this happens over WiFi, as this is not expected.

Test: atest FrameworksNetTests
Test: Manually inspect logs and output of `dumpsys connmetrics`

Bug: 276498460
Change-Id: Iba20ec1f47b682cfff6127eeade0fdbf912945a6