Prevent NPE when session tracker has not been initialized

[mzorz]

Fixes #9494

Added null check as tracker may not have been initialized if site or Post were null in the first place and Editor was exited preemptively

To test:

1. artificially make Post or Site null in onCreate(), or add a call to showErrorAndFinish() before the call to createPostEditorAnalyticsSessionTracker()
2. run and open the Editor
3. observe it exits but doesn't crash

Update release notes:

• If there are user facing changes, I have added an item to RELEASE-NOTES.txt.

[mzorz]

hi @jtreanor - let me know if the base branch needs be changed, we can create another branch and cherry pick it branching off from release/12.1 if needed 👍

[jtreanor]

Thanks for the quick fix @mzorz!

My preference would be to target release/12.1 with this. I tried changing the base but this branch is ahead. Cherry picking once merged is also fine.

[jtreanor]

This looks good. Thanks Mario!

I'll let you decide if you want to change base to release/12.1 or merge and cherry pick.