fix: ensure that generated dts is compatible with --strictNullChecks

[forivall]

Updates src/Responses.ts to ensure compatibility with --strictNullChecks

Without this change, when using this library with --strictNullChecks, typescript will throw the build error:

dist/Responses.d.ts(4,5): error TS2411: Property 'default' of type 'Reference | Response | undefined' is not assignable to string index type 'Reference | Response'.

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