Reverted the ApnSetting behavior

Returning null is already a behavior mentioned in the
public API document. Reverted the behavior back to be consistent
with Android 12.

Fix: 201089892
Test: atest CtsDevicePolicyManagerTestCases
Merged-In: Id4c8b41b23adfe931b9953113f7175a11703dc10
Change-Id: Id4c8b41b23adfe931b9953113f7175a11703dc10