Fix NullPointerException on singleUserSite access

[zwarm]

Fixes #19518
Sentry Issue: WORDPRESS-ANDROID-2S4V

This fixes a NullPointerException crash while checking the value of site.isSingleUserSite. This PR addresses the immediate crash; however how the boolean value is getting set to null is still needs investigation.

Tested

• New site creation sets a non-null value in the db
• Migrating the db sets a non-null value in the db
• gson setting false

cc: @oguzkocer

To test:

• Install the app
• Login and select a site
• Navigate to MySite > More > Posts
• ✅ Validate the post list loads as normal and the author filter functions as expected

If you want to force the null

• Download this branch
• Download the associated FluxC branch
• Set FluxC to build locally within WP (local-builds.config)
• Navigate to siteResponseToSiteModel within SiteRestClient
• Change the following line from site.setIsSingleUserSite(from.single_user_site) to site.setIsSingleUserSite(null)
• Build and launch the app
• In Android Studio, Navigate to DatabaseInspector > SiteModel
• Notice that IsSingleUserSite = null
• In the app, navigate to MySite > More > Posts
• ✅ Validate the post list loads as normal

Regression Notes

1. Potential unintended areas of impact
   The posts list still crashes and the author filter does not work properly

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   Add new unit test to PostListMainViewModelTest

3. What automated tests I added (or what prevented me from doing so) N/A

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.

UI Changes testing checklist: N/A

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Flavor	Jalapeno
	Build Type	Debug
	Version	pr19523-21c5e73
	Commit	21c5e73
	Direct Download	jetpack-prototype-build-pr19523-21c5e73.apk

Note: Google Login is not supported on these builds.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress by scanning the QR code below to install the corresponding build.

	App Name	WordPress
	Flavor	Jalapeno
	Build Type	Debug
	Version	pr19523-21c5e73
	Commit	21c5e73
	Direct Download	wordpress-prototype-build-pr19523-21c5e73.apk

Note: Google Login is not supported on these builds.

[peril-wordpress-mobile]

	Warnings
⚠️	This PR is assigned to a milestone which is closing in less than 4 days Please, make sure to get it merged by then or assign it to a later expiring milestone

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[AjeshRPai]

👍🏼 LGTM