Invalid JSON emitted from `swiftly use --format=json` with no installed or selected toolchains

[etcwilde]

swiftly use --format=json returns empty string "" (not to be confused with '""' ), when there are no selected toolchains or installed toolchains:

%  swiftly use --format=json
%

swiftly/Sources/Swiftly/Use.swift

Lines 90 to 93 in d166ae1

	guard let selectedVersion else {
	// Return with nothing if there's no toolchain that is selected
	return
	}

No-output is not valid JSON, which trips up tools like the VSCode plugin. An empty object would likely be appropriate here instead.
I can get into this state if I run swiftly uninstall 6.3.2 (pre-installed toolchain) and not having any other toolchains installed and managed by Swiftly.

[etcwilde]

Misbehaving config.json:

{
  "installedToolchains" : [
  ],
  "platform" : {
    "namePretty" : "macOS",
    "nameFull" : "osx",
    "name" : "xcode"
  },
  "version" : "1.1.1"
}