Problem

When using a plugin to disable Custom Fields in the post editor, WordPress attempts to change $editor_settings to tell Gutenberg not to render the Custom Fields settings toggle in a preferences modal. However, Gutenberg is listening for a value that is impossible to send in PHP, and the settings toggle remains visible even when the feature itself is disabled.

This change makes it so when PHP uses unset() to not send a value, then undefined is the value, allowing the field to be hidden.

Steps to reproduce and more context

Please see #33912. This is tackling the same problem but in a different way.