Adds additional check to make sure the show_in_rest parameter is present before it is manipulated

[thijsoo]

…

What?

This pr makes sure that it is checked that show_in_rest is present before it is manipulated and shows a notice for plugins using register_setting without specifying show_in_rest.

Why?

This solves #60859

How?

I have added an additional isset check.

Testing Instructions

1. Install Gutenberg latest version and & Yoast SEO
2. Go to Yoast SEO->Settings
3. Confirm that you don't get a Warning: Undefined array key "show_in_rest" warning

Testing Instructions for Keyboard

Screenshots or screencast

[github-actions]

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

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[github-actions]

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[github-actions]

👋 Thanks for your first Pull Request and for helping build the future of Gutenberg and WordPress, @thijsoo! In case you missed it, we'd love to have you join us in our Slack community.

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[thijsoo]

@aristath I heard that you would be willing to take a look

[Mamaduka]

Sorry, @thijsoo. I didn't know you were already working on this and created fix for the same issue - #60862.

[thijsoo]

Sorry, @thijsoo. I didn't know you were already working on this and created fix for the same issue - #60862.

No problem! Just happy to see the notice fixed :).

[Mamaduka]

I'll make sure to include you in the list of props. Sorry again for the confusion.