Fixes #12880
Shared PR: wordpress-mobile/WordPress-iOS-Shared#249

To test:

1. Launch the app

2. If you're logged in, log out

3. Tap Continue with WordPress.com

4. Login to an account that has no sites

5. When the PSI is displayed, verify this in the console:
   🔵 Tracked: welcome_no_sites_interstitial_shown

6. Tap the create site button, and verify this in the console:
   🔵 Tracked: welcome_no_sites_interstitial_button_tapped <button: create_new_site>

7. Tap the add self-hosted site button, and verify this in the console:
   🔵 Tracked: welcome_no_sites_interstitial_button_tapped <button: add_self_hosted_site>

8. Tap the Not Right Now option, and verify this in the console:
   🔵 Tracked: welcome_no_sites_interstitial_dismissed

Note: In order to test each button option you will need to either:

• Delete and relaunch the app after each button press since the PSI will not be displayed again for your account.
• Login using a different account for each action

PR submission checklist:

• I have considered adding unit tests where possible.
• 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.