Allow nullish actions

[kwangure]

Describe the problem

I'm frustrated when I have to use a noop action, as a placeholder while Svelte accepts nullish values in other places in the template.

Describe the proposed solution

Do not error when nullish actions are used.

<script>
	const foo = undefined;
</script>

<button onclick={foo}>foo</button>
<button class:foo>bar</button>
<!-- Error: foo is not a function  -->
<button use:foo>bar</button>

Importance

would make my life easier

[Prinzhorn]

AFAIK actions are not reactive at all right now, so even if nullish was supported, you couldn't swap it with an actual implementation later:

#6754
#6942

[kwangure]

This is helpful if you're passing nullable actions as props.

[Prinzhorn]

This is helpful if you're passing nullable actions as props.

I didn't say it wasn't helpful. But actions cannot be swapped out right now (see linked issues). So if your prop changes to non-nullish later, it won't update.

[dummdidumm]

Closing in favor of #6942