Ensure valid application pointer in RAI::CheckLanguage and FinishSendingResponseToMobile

[iCollin]

Fixes #3813

This PR is ready for review.

Risk

This PR makes no API changes.

Testing Plan

https://github.com/smartdevicelink/sdl_atf_test_scripts/pull/2602

Summary

Prevent derefing nullptr for application in some methods during registration

CLA

• [x] I have signed the CLA

[iCollin]

Yes good theory, we should also probably check if application has been unregistered before beginning ResumptionDataProcessorImpl::Restore.