fix: don't throw with nullish actions

[paoloricciuti]

Svelte 5 rewrite

Closes #13555 ...this may need some discussion because i don't know if it was done on purpose but it was simple enough i said, why not.

Also working on this i noticed we are creating an unnecessary thunk where we could just pass the action in (especially since it's not reactive)...should we fix this too?

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[Prinzhorn]

Did you see #6942 I've linked? If actions aren't reactive, what's the point of supporting nullish actions? That's a noop. If the prop later changes to non-nullish, the action will still be nullish. See https://svelte-5-preview.vercel.app/#H4sIAAAAAAAAA52SUWvEIAzHv4qTg1o47t57bWHfYW_rPVibFpmnRePgKH732dprxzbGGCIm-cf4I3GivVTgaPE6Uc1vQAv6PI70SPE-zo57B4UQfWe8FXOkdMLKEetGN6gACZphUEAqcnDIEVjPlYP88pC5QGn0LvNFimLvdZI406aDnExztEFhtDMKTsoMLOPZkSxqvESSbgG91Y_sBjtwaM2d5XvohyLZmpYlsnmFZCxH-MLU_sLUfmL6L1L7Z6SD1G4EgSw1cs4tz_sIdNl6xEjsHRRrr40WSoq3aooAVZ0Ytik9JSs9GbfsWYpssNvE2iUpEIgD_SbyywoZQv2yFCjPCSVixf9yM53sJXS0QOshXMMHiz-npmoCAAA=

[paoloricciuti]

Did you see #6942 I've linked? If actions aren't reactive, what's the point of supporting nullish actions? That's a noop. If the prop later changes to non-nullish, the action will still be nullish. See https://svelte-5-preview.vercel.app/#H4sIAAAAAAAAA52SUWvEIAzHv4qTg1o47t57bWHfYW_rPVibFpmnRePgKH732dprxzbGGCIm-cf4I3GivVTgaPE6Uc1vQAv6PI70SPE-zo57B4UQfWe8FXOkdMLKEetGN6gACZphUEAqcnDIEVjPlYP88pC5QGn0LvNFimLvdZI406aDnExztEFhtDMKTsoMLOPZkSxqvESSbgG91Y_sBjtwaM2d5XvohyLZmpYlsnmFZCxH-MLU_sLUfmL6L1L7Z6SD1G4EgSw1cs4tz_sIdNl6xEjsHRRrr40WSoq3aooAVZ0Ytik9JSs9GbfsWYpssNvE2iUpEIgD_SbyywoZQv2yFCjPCSVixf9yM53sJXS0QOshXMMHiz-npmoCAAA=

To support components where you might or might not pass the prop.

<script>
    const { action, children, ...rest } = $props();
</script>

<button use:action {...rest}>
    {@render children?.()}
</button>

You can use it

<Button title="look ma no action">click</Button>

Without having to declare a noop

[Prinzhorn]

Fair point, but to me that sounds like a footgun though when we support nullish actions but you cannot update them.
But on the other hand the problem already exists with two actions that are non-nullish and the prop change won't be reflected. So yeah, ignore my comment.