Fix the potential NPE when calling IPMS API and ExecutionException is thrown.

Listener callback argument is marked as @Nullable in some APIs, means
client isn't interested in learning about return value, in these cases
should check if it's null or not before using this callback.

Bug: 147922038
Test: atest NetworkStackTests FrameworksNetTests
Change-Id: Ib46624c76c2c1e50c73ccfbb295a0e90e6fa1c88