Settings: Post: Defaults: Use 0 instead of blank string

[n7studios]

Summary

This mirrors how settings are saved when adding/editing a Page, Post or CPT in WordPress and setting Landing Page, Add a Tag and/or Member Content functionality to ‘None’ / ‘Don’t restrict’, which would store these settings as 0, not a blank string.

Required for the Abilities API / MCP schema, so we consistently use '0' when setting 'None' values on a Post's settings.

Testing

Existing tests pass.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)