Fix PHP notice triggered by 'gutenberg_update_initial_settings'

[Mamaduka]

What?

Fixes #60859.

PR updates logic in the gutenberg_update_initial_settings filter callback to bail out early when the setting isn't exposed via REST API.

Testing Instructions

1. Register a setting without show_in_rest.
2. Load the testing site.
3. Confirm that the following warning isn't displayed or logged - PHP Warning: Undefined array key "show_in_rest"

Example setting

add_action( 'init', function() {
	register_setting(
		'general',
		'foo_my_setting',
		array(
			'type'              => 'string',
			'label'             => 'My Setting',
			'description'       => 'This is a custom setting.',
		)
	);
} );

Testing Instructions for Keyboard

Same.

Screenshots or screencast

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Co-authored-by: Mamaduka <[email protected]>
Co-authored-by: leonidasmi <[email protected]>

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