Fix crash for mismatched account case

[irfano]

(cherry picked from commit 8219ffa)

Description

This fixes a crash on the site picker screen when the sites data is null. I couldn't reproduce the crash. But when the sites.value is null for any reason, it makes sense to hide the primary button on the site picker screen.
This case may occur rarely and temporarily when the screen is initializing.

Steps to reproduce

"To reproduce the issue, you need a WordPress.com account with 0 sites. Not even a blog. Which is tricky to get through the normal flow because WP.com will create 1 blog for you as soon as you create a new account. However, if you delete that blog (and wait a few hours until the deletion is completed) you'll end up with a WP.com account with 0 sites. Using that account o log into any Jetpack connected site will result in the account mismatch screen and the app crashing. - #12889 (comment)"

Testing information

Tested normal mismatch account case.

The tests that have been performed

Tested normal mismatch account case again for this PR.

• I have considered if this change warrants release notes and have added them to RELEASE-NOTES.txt if necessary. Use the "[Internal]" label for non-user-facing changes.

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• The PR is small and has a clear, single focus, or a valid explanation is provided in the description. If needed, please request to split it into smaller PRs.
• Ensure Adequate Unit Test Coverage: The changes are reasonably covered by unit tests or an explanation is provided in the PR description.
• Manual Testing: The author listed all the tests they ran, including smoke tests when needed (e.g., for refactorings). The reviewer confirmed that the PR works as expected on big (tablet) and small (phone) in case of UI changes, and no regressions are added.

[hichamboushaba]

Looks good, the changes were already tested in #12889

[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 21.0.1. The due date for this milestone has already passed. Please assign it to a milestone with a later deadline or check whether the release for this milestone has already been finished.

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	App Name	WooCommerce-Wear Android
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	Commit	cab9473
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[wpmobilebot]

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

	App Name	WooCommerce Android
	Platform	📱 Mobile
	Flavor	Jalapeno
	Build Type	Debug
	Commit	cab9473
	Direct Download	woocommerce-prototype-build-pr12914-cab9473.apk