Switching checked to null should leave the current value (#26667)

I accidentally made a behavior change in the refactor. It turns out that
when switching off `checked` to an uncontrolled component, we used to
revert to the concept of "initialChecked" which used to be stored on
state.

When there's a diff to this computed prop and the value of props.checked
is null, then we end up in a case where it sets `checked` to
`initialChecked`:


https://github.com/facebook/react/blob/5cbe6258bc436b1683080a6d978c27849f1d9a22/packages/react-dom-bindings/src/client/ReactDOMInput.js#L69

Since we never changed `initialChecked` and it's not relevant if
non-null `checked` changes value, the only way this "change" could
trigger was if we move from having `checked` to having null.

This wasn't really consistent with how `value` works, where we instead
leave the current value in place regardless. So this is a "bug fix" that
changes `checked` to be consistent with `value` and just leave the
current value in place. This case should already have a warning in it
regardless since it's going from controlled to uncontrolled.

Related to that, there was also another issue observed in
https://github.com/facebook/react/pull/26596#discussion_r1162295872 and
https://github.com/facebook/react/pull/26588

We need to atomically apply mutations on radio buttons. I fixed this by
setting the name to empty before doing mutations to value/checked/type
in updateInput, and then set the name to whatever it should be. Setting
the name is what ends up atomically applying the changes.

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Co-authored-by: Sophie Alpert <[email protected]>