Fix crash in visitEachChildOfIndexSignatureDeclaration when type is missing

[mango766]

Fixes #63230

Problem

When a malformed index signature like {[key: string], any} (comma instead of colon) is parsed, the IndexSignatureDeclaration node is created without a type property. This is expected — the parser already handles this case, and there's even a TODO comment in createIndexSignature acknowledging it:

node.type = type!; // TODO(rbuckton): We mark this as required in IndexSignatureDeclaration, but it looks like the parser allows it to be elided.

However, visitEachChildOfIndexSignatureDeclaration uses Debug.checkDefined(nodeVisitor(node.type, ...)) which throws a Debug Failure instead of allowing the compiler to report a proper diagnostic error.

Fix

Replace Debug.checkDefined(...) with the non-null assertion operator (!), matching the existing pattern used by createIndexSignature for the same situation. When node.type is undefined, nodeVisitor returns undefined, and the ! allows it to propagate through updateIndexSignature — which already handles the case gracefully (it compares node.type !== type, sees they're both undefined/the same, and returns the original node unchanged).

Minimal repro (from @RyanCavanaugh's comment)

export const f = (x: {[key: string], any}) => 0;

Before: Error: Debug Failure.
After: proper diagnostic errors are reported.

[typescript-bot]

With 6.0 out as the final release vehicle for this codebase, we're closing all PRs that don't fit the merge criteria for post-6.0 patches. If you think this was a mistake and this PR fits the post-6.0 patch criteria, please post to the 6.0 iteration issue with details (specifically, which PR and which patch criteria it satisfies).

Next steps for PRs:

• For crash bugfixes or language service improvements, PRs are currently accepted at the typescript-go repo
• Changes to type system behavior should wait until after 7.0, at which point mainline TypeScript development will resume in this repository with the Go codebase
• Library file updates (lib.d.ts etc) continue to live in this repo or the DOM Generator repo as appropriate