dataset optimization does not respect null indicating to remove the attribute

[Conduitry]

Describe the bug
data-foo={bar} should, like other attribute settings, remove the attribute when setting it to a value of null or undefined. The optimization (used unless legacy is enabled) of instead assigning to the dataset object prevents this.

To Reproduce

<script>
	let name = 'world';
	let toggle = false;
</script>

<input type='checkbox' bind:checked={toggle}>

<h1 data-foo={toggle ? name : null}>Hello {name}!</h1>

Expected behavior
The data-foo attribute should be not present when the checkbox is unchecked, not the string 'null'.

Severity
Not personally affecting me.

Additional context
Came up in regard to this question. I believe this should be the way to handle that, but it doesn't work.

[Conduitry]

I'm not sure the extent that #858 was intended as a speed optimization vs a size optimization. The simplest solution would be to remove that optimization (to bring this in line with the other recent move to do almost everything with attributes instead of properties), but I don't know what that would lose.

[Rich-Harris]

If it was intended as a speed optimisation it was misguided:

I think it was a size thing more than anything. Dropping it seems like the straightforward option. With that change, legacy would only be used for readonly input.type properties and erroring on once/passive event modifiers.

[Conduitry]

Sounds good - I'll prepare a PR