Swiftly-Use: JSON: Handle no selected toolchain (#546)

Uninstalling all of the tools in Swiftly and printing the current
toolchain with `swiftly use --format=json` results in swiftly printing
nothing. This is invalid JSON, so tools like the VSCode extension would
emit an error.

Without the fix, the empty JSON passed to the Foundation JSON decoder
emits the following error:

```
Caught error: Error Domain=NSCocoaErrorDomain Code=3840 "Unable to parse empty data." UserInfo={NSDebugDescription=Unable to parse empty data.}
```

Fixes: #545

Author: etcwilde

Sources/Swiftly/OutputSchema.swift

@@ -1,6 +1,10 @@
 import Foundation
 import SwiftlyCore
 
+struct EmptyObject: OutputData {
+    var description: String { "" }
+}
+
 struct LocationInfo: OutputData {
     let path: String
 

Sources/Swiftly/Use.swift

@@ -89,6 +89,7 @@ struct Use: SwiftlyCommand {
 
             guard let selectedVersion else {
                 // Return with nothing if there's no toolchain that is selected
+                try await ctx.output(EmptyObject())
                 return
             }
 

Tests/SwiftlyTests/UseTests.swift

@@ -351,6 +351,20 @@ import Testing
             }
     }
 
+    /// Tests that `swiftly use --format=json` emits a valid empty JSON object when no toolchain is selected.
+    @Test(.mockedSwiftlyVersion(), .mockHomeToolchains(toolchains: []))
+    func printInUseJsonFormatNoSelection() async throws {
+        let output = try await SwiftlyTests.runWithMockedIO(
+            Use.self, ["use", "--format", "json"], format: .json
+        )
+
+        let joined = output.joined(separator: "\n")
+        let data = try #require(joined.data(using: .utf8))
+        let json = try JSONSerialization.jsonObject(with: data)
+        let object = try #require(json as? [String: Any])
+        #expect(object.isEmpty)
+    }
+
     /// Tests that running a use command without an argument prints the currently in-use toolchain.
     @Test(.mockedSwiftlyVersion()) func printInUseJsonFormat() async throws {
         let toolchains = [