chore: fix return type for WP_Duotone_Gutenberg::get_selector()

[justlevine]

What?

This PR fixes the @return type on WP_Duotone_Gutenberg::get_selector() to correctly indicate that it possibly returns null.

Additionally, an explicit return null has been added to the end of function, instead of coercing the implicit void if an invalid block name is passed to the method.

Why?

While this issue was surfaced by PHPStan (via #66693) it can be remediated independently as part of #66598

How?

Testing Instructions

Testing Instructions for Keyboard

Screenshots or screencast

[github-actions]

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[github-actions]

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[github-actions]

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[gziolo]

Thank you for catching it and the fix 👍🏻