[fix] avoid infinite loop if no routes found

[baseballyama]

fix: #2602

Please refer My Comment for details.
I handled only src/routes/index.svelte because if another file is not found, we can notice it on browser.
(We can see a 404 message on browser.)

(Is it correct that always src/routes/index.svelte exists?🤔)
-> According to the comment, may not have src/routes/index.svelte.

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[Conduitry]

This isn't the right solution. The root path might be served by an endpoint, or it might not exist at all (especially if the SvelteKit app is behind something that only sends certain URLs to it), or the user may simply be using a different file extension for their page routes.

[baseballyama]

@Conduitry

Thank you for telling point what I'm not aware of.
And I updated code.

The direct cause is an infinity loop here.

kit/packages/kit/src/core/create_app/index.js

Lines 132 to 142 in 26e81e8

	while (l--) {
	pyramid = `
	<svelte:component this={components[${l}]} {...(props_${l} || {})}>
	{#if components[${l + 1}]}
	${pyramid.replace(/\n/g, '\n\t\t\t\t\t')}
	{/if}
	</svelte:component>
	`
	.replace(/^\t\t\t/gm, '')
	.trim();
	}

So I updated it to prevent infinity loop.

And if none of the route components are present, this infinity loop comes.
But I'm not sure that this is a possible use case for kit users.

So right now I added warn message if there is no route component.

[dummdidumm]

Maybe add a little comment explaining what max_depth of 1 then there are actually 0 routes means? It's not immediately obvious that this is produces an error somewhere else as a consequence.