Fix fieldset legend displaying undefined

[seancolsen]

Fixes #4091

Notes

This fixes a regression that was probably introduced by #3987, or #4073, or a combination of both.

I hate the way Svelte 3 deals with slot forwarding. There are weird foot guns lurking when composing components that take slots. I think Svelte 5 clears this up via snippets, and I'm very excited about it!

For now, the fix in this PR seems like the cleanest way to handle it, even though I don't love it.

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