fix: improve safety when resolving accessor types

[Divi2709]

Fix unsafe access when resolving accessor types in processor.
Previously, accessor type resolution assumed that getSignature or setSignature would always be defined. This could lead to runtime errors when these properties were undefined.
This change adds optional chaining to safely access nested properties and preserves existing comments when signature comments are missing.
The output format remains unchanged, ensuring compatibility with doc-kit.

[linux-foundation-easycla]

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• ✅ login: Divi2709 / name: Divija (3215d0d)

[avivkeller]

Has this error ever occurred? Given the context, these will always be defined, no?

[udaykumarreddykasaram07-eng]

I looked at the processor.mjs code I think avivkeller's question is valid. Since getSignature and setSignature come from TypeDoc's Declarationreflection, they would only exist on actual accessor declarations.It might be worth checking if there is a real case in webpack's types.d.ts where this would be undefined before merging.