Fix editor plugin error on CPTs without custom-fields support

[pfefferle]

Summary

• Return null early if meta is undefined, which happens when the custom post type doesn't support custom-fields
• Add fallback to false for the checkbox checked values to handle undefined meta properties

Test plan

• Create a custom post type without custom-fields support but with webmentions support
• Verify no error banner appears in the block editor
• Verify the Webmentions panel doesn't render (since meta is unavailable)

Fixes #529

[Copilot]

Pull request overview

This PR fixes an error that occurs when the Webmentions editor plugin is used with custom post types (CPTs) that don't support custom-fields. When a CPT lacks this support, the meta property becomes undefined, causing runtime errors in the block editor.

Key Changes:

• Added an early return when meta is undefined to prevent errors
• Added || false fallback values to checkbox checked properties (though these are redundant given the early return)

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/editor-plugin/plugin.js	Adds early return check for undefined meta and fallback values for checkbox properties to prevent errors on CPTs without custom-fields support
readme.md	Documents the bug fix in the 5.5.1 changelog entry

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