[gRPC] Neither `merge` nor `setValue` can be used to unset a CLI config entry

[kittaakos]

Bug Report

Current behavior

I can use neither merge nor setValue to unset a property in the CLI config when running in daemon mode.

Let assume I have the following state that is loaded by the ArduinoCoreClient, and I want to unset proxy.

{
  "foo": "bar",
  "proxy": "http://blabla"
}

I tried to update the CLI state with the followings:

{ "network": { "proxy": undefined } }

{ "network": { } }

Neither merge nor setValue worked.

Expected behavior

I can unset a value.

Environment

• CLI version (output of arduino-cli version): 0.15.0-rc1
• OS and platform:

Additional context

This feature is required by arduino/arduino-pro-ide#410