Default Randomized MAC Address to 0:0:0:0:0:0 in WifiConfig

Set the default value of Randomized MAC address as 0:0:0:0:0:0 instead
of null.

Making this change to simplify storing randomized address to XML for
permanent storage. Without this change, will need to manually check if
the value is null during MacAddress<->XML conversion.

Bug: 72753415
Test: Unittest.
Change-Id: I6edf486360e267acec9f8a343386444be2ea99fa