SwitchManager.getHardwareMAC() does not handle empty string case

1. Start OpenDaylight on a Windows machine
2. Start Mininet using a tree topology of level 2, and specify the IP address of your OpenDaylight instance as the remote controller. Mininet will create a topology with 1 switch and 2 hosts connected to it.
3. Log in to OpenDaylight and view the topology UI

Bug: The UI shows the switch, but not the hosts created in Mininet

Bug: Also, if you do a pingall in Mininet, it will always fail even after repeated attempts.

My colleague thinks the issue may be in SwitchManager.java SwitchManager.getHardwareMAC() where the macAddress value is checked for null but not for empty string.