Do not throw InvalidArgumentException.

If subId is invalid, getSubscriptionUserHandle throws
InvalidArgumentException which results in a crash. Instead, return null
when subId is invalid.

Bug:289137152
Test: Flashed build on raven-userdebug and performed basic functionality tests,
- atest SubscriptionManagerServiceTest
- QA confirmed that crash is not reproduced with the fix in b/289137152#comment13

Change-Id: Ib4f4972c931c019558e3c5459c4431cb4b124524