Afform - Add "clear" button to non-required radios

[colemanw]

Overview

Fixes dev/core#3450
Allows the value of radio buttons to be unset if the field is not required, consistent with the rest of the CiviCRM UI.

Before

No way to clear radio buttons.

After

[civibot]

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[artfulrobot]

Thanks @colemanw

r-run done on 5.47.4 site (patch applied cleanly).

• ✔ no default, not required: radios show both unselected. Click one, filter works. Click the other, same. Click the ✖ and they get reset to null. All good.
• ✔ no default, required: radios show both unselected. Click one, filter works. Click the other, same. There is now no ✖. All good.
• ⚠️ default 'yes', not required: radios show both unselected, but with the clear button also. Also, the clear button appears if I select Yes, but disappears if I select No.
• ⚠️ default 'no', not required: radios show both unselected, but with the clear button also. Also, the clear button appears if I select Yes, but disappears if I select No.
• ⚠️ default 'yes' or 'no, and required: radios show both unselected (but I think the value is passed to the filter). Correctly, there is no clear button.

EDIT: note that in my testing I have been using a boolean field; this may differ from your screenshot where options were used, since for options "false" would be correct since that would be a label/name (were it an option value) as opposed to meaning boolean false.

[eileenmcnaughton]

@colemanw looks like @artfulrobot has given this pretty thoughtful review - please self-merge this when you get @artfulrobot's go ahead

[eileenmcnaughton]

@colemanw did you have a chance to look

[colemanw]

I fixed the issue with the button showing up for "Yes" but not "No". As you might guess it was because false == null so I had to do type checking there.
The other issue is not directly related to this PR so I'm going to fix it separately.