fix: type mismatch for get and set methods

[qoog]

Summary

Fixed type mismatches for get and set methods:

• get now returns null instead of undefined to match string | null.
• set now accepts a single string argument, even if the body is empty.
  These changes prevent TypeScript errors and ensure the API aligns with its type definitions.

How did you test your changes?

TypeScript compilation passes without errors. Verified that get and set calls behave as expected in the consuming code.

[cb-heimdall]

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