test: add test case to libjxl.test.ts when input and output are not jxl

tests/converters/libjxl.test.ts

@@ -82,3 +82,29 @@ test("convert uses cjxl with output filetype being jxl", async () => {
   expect(command).toEqual("cjxl");
   expect(loggedMessage).toBe("stdout: Fake stdout");
 });
+
+test("convert uses empty string as command with neither input nor output filetype being jxl", async () => {
+  const originalConsoleLog = console.log;
+
+  let loggedMessage = "";
+  console.log = (msg) => {
+    loggedMessage = msg;
+  };
+
+  const mockExecFile: ExecFileFn = (
+    _cmd: string,
+    _args: string[],
+    callback: (err: ExecFileException | null, stdout: string, stderr: string) => void,
+  ) => {
+    command = _cmd;
+    callback(null, "Fake stdout", "");
+  };
+
+  const result = await convert("input.png", "png", "jpg", "output.jpg", undefined, mockExecFile);
+
+  console.log = originalConsoleLog;
+
+  expect(result).toBe("Done");
+  expect(command).toEqual("");
+  expect(loggedMessage).toBe("stdout: Fake stdout");
+});