`jamulusclient/setSkillLevel` with `null` returns a response with neither `result` nor `error`

[mcfnord]

🤖 AI: Split out of #3884 at pljones's request — the one of the three that isn't a range check.

null is a documented-valid value that clears the skill level. Its branch (src/clientrpc.cpp:304) sets SL_NOT_SET, calls SetRemoteInfo(), and returns at src/clientrpc.cpp:308 without assigning response["result"]:

skillLevel	response
null	{"id":4,"jsonrpc":"2.0"}
"expert"	{"result":"ok"}

A response with neither member violates JSON-RPC 2.0 §5, and the method's own doc says @result — Always "ok". A strict client waiting for result blocks on a valid call. Fix: add response["result"] = "ok"; before the return.

Measured on main @ 8b667a3a, Qt 5.15.3, x86-64, QT_QPA_PLATFORM=offscreen; line numbers re-verified unchanged against origin/main @ 11a28d57e33506fc84cb9b5ee02097931c1e35a3 on 2026-08-21.

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🤖 This message was written by AI and reviewed by @mcfnord.