Fix NullPointerException on NsdService

The NsdService will throw a NPE if a new client is registered
with a null INsdManagerCallback object. To avoid this, perform
a null check before registering a new client and throw an
IllegalArgumentException if the callback is null.

Bug: 293285797
Test: atest FrameworksNetTests
Change-Id: Id61e27873591031c3fe383879aee0d40eebc08b3