Fixes IllegalArgumentException due to uninitialised site in page template picker

[antonis]

Fixes #15408

To test:

• I don't have consistent steps to trigger the reported crash. A way to test this may involve using the debugger to set a null value for the SiteModel.
• Verify that the tests pass

Regression Notes

1. Potential unintended areas of impact
   N/A

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   N/A

3. What automated tests I added (or what prevented me from doing so)
   Added a test case

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[antonis]

Hey @ParaskP7, @planarvoid 👋
I've added you as reviewers since you seem to be familiar with this part of the codebase and handling the case where the SiteModel has not been initialised. Any suggestions for improvements or alternative approaches are more than welcome 🙇

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading the APKs:

• Jetpack-pr-15415-build-116140.apk
• WordPress-pr-15415-build-116140.apk

[ParaskP7]

👋 @antonis !

I've added you as reviewers since you seem to be familiar with this part of the codebase and handling the case where the SiteModel has not been initialised. Any suggestions for improvements or alternative approaches are more than welcome 🙇

Thanks for creating the fix, I am on it! 🙏

[ParaskP7]

👋 @antonis !

I have reviewed and tested this PR as per the instructions, everything works as expected, good job! 🌟

I have left one suggestion (💡) for you to consider. I am going to approve this PR anyway, since none is blocking. I am NOT going to merge this PR yet to give you some time to think about and maybe apply this suggestions. However, feel free to ignore them and merge the PR yourself.

Extra Suggestion (💡): Consider making the SiteModel site within the SiteUtils.isBlockEditorDefaultForNewPost(...) method explicitly @Nullable to make it explicit that this method can accept a nullable site.

[ParaskP7]

👋 @antonis !

Thanks for your updates, those LGTM, I am still on the approved side, feel free to merge when CI completes.