When a network preference is set, the highest priority nri will
be a managed default request that disallows default networking.
In the case where there is no satisfying network,
mNoServiceNetwork is used as the satisfier instead of null.
(see computeNetworkReassignment)

mNoServiceNetwork should not be returned in any public API.
Check for the nri being satisfied before returning the satisfier
to ensure mNoServiceNetwork is not returned.

Fixes: 301222648
Test: atest FrameworksNetTests
Change-Id: I22d67a7e8d0274d8ad4f6123fbedf6d37eed18e7