fix: remove role attribute when it's null or undefined

[DAreRodz]

Fixes #4136.

Most browsers now implement the attributes defined in the ARIAMixin interface, which includes Element.role. In such a case, the current code assigns an empty string to the role property when it's null or undefined instead of removing it.

This PR prevents that so the role prop can be removed as normal.

[github-actions]

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[coveralls]

coverage: 99.294%. remained the same when pulling eb367fe on DAreRodz:fix-role-attribute-removal into 7dc66bc on preactjs:main.

[JoviDeCroock]

Awesome, thank you!