code quality: explicitly return null in wp_get_nav_menu_to_edit()

[justlevine]

This PR updates wp_get_nav_menu_to_edit() to explicitly return null and adds null to the function's return type. This is instead of the existing behavior where the implict void return is not typehinted.

While this issue was surfaced via PHPStan in #7619 (trac: https://core.trac.wordpress.org/ticket/61175 ), it can be remediated independently of that ticket.

Trac ticket: https://core.trac.wordpress.org/ticket/63268

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[SergeyBiryukov]

Thanks for the PR! Merged in r60180.