fix: adjust missing inner content warning

[ignatiusmb]

Alright, v2 of #11380

Svelte 5 can still use <slot> and +layout can now accept "named slots" with snippets. I think we can safely assume some content will be rendered, as long as a {@render tag exists in the markup, without having to check which snippet(s) also exists in the script.

Closes #11127

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