Fix NPE in restoring network policies

When backing up network policies, we write null for policies that are
null or are inferred. When restoring, this sends null policies to be set
which results in a NPE.

Bug: 79961560
Test: 1) atest NetworkPolicyManagerServiceTest
2) Manual: adb restore using backup set with null policies does not
crash

Change-Id: I450685e38acae0658ea984b86ca8b17ca27a71a6