fix: allow falsy values for render tags

#2319

Author: dummdidumm

packages/svelte2tsx/svelte-shims-v4.d.ts

@@ -146,7 +146,7 @@ declare function __sveltets_2_nonNullable<T>(type: T): NonNullable<T>;
 declare function __sveltets_2_cssProp(prop: Record<string, any>): {};
 
 // @ts-ignore Svelte v3/v4 don't have this
-declare function __sveltets_2_ensureSnippet(val: ReturnType<import('svelte').Snippet>): any;
+declare function __sveltets_2_ensureSnippet(val: ReturnType<import('svelte').Snippet> | undefined | null): any;
 // @ts-ignore Svelte v3/v4 don't have this
 declare function __sveltets_2_snippet(): import('svelte').Snippet;
 declare function __sveltets_2_snippet<T>(t: T): import('svelte').Snippet<[T]>;