Avoid system_server crashing due to mac address is null

It's possible that the mac address is null (getMacAddress() of WifiNative.java).
In this case system_server will crash like:

E AndroidRuntime: *** FATAL EXCEPTION IN SYSTEM PROCESS: WifiWatchdogStateMachine
E AndroidRuntime: java.lang.NullPointerException
E AndroidRuntime:        at android.net.arp.ArpPeer.<init>(ArpPeer.java:57)
E AndroidRuntime:        at android.net.wifi.WifiWatchdogStateMachine.doArpTest(WifiWatchdogStateMachine.java:866)

Check the mac address before parsing to avoid crashing.

Change-Id: I5d4205c04d479a3a2837172c6382816ea4bf74d6