sdk: handle 204 No Content in writeApiResult

[chayu163]

What changed

Added a 204 branch to ZSClient.writeApiResult so a 204 No Content
response is treated as a successful empty result instead of falling through to
the unknown status code exception.

Why the change is needed

doCall already treats 200 || 204 as success and calls writeApiResult for
both. Because writeApiResult only handled 200 and 503, a 204 response
would throw ApiException("unknown status code: 204"), turning a successful
call into an error.

How the change was tested

Incremental one-branch addition; compiles cleanly. Not verified against a
running server.

Related issues

Fixes #52

Compatibility / upgrade impact

None — only adds handling for a previously unhandled success status code.