Allowing for null net caps in updateConfiguration

Marking NetworkCapabilities as nullable in updateConfiguration and
updating where needed to support this. This will allow callers of the
ethernet network management updateConfiguration API to use it primarily
for setting an ethernet network's IP configuration.

Bug: 222565654
Bug: 220017952
Bug: 210485380
Test: atest EthernetNetworkUpdateRequestTest

Change-Id: I1f80e950acbd078a6b3378e1fa5f1e4173112e79