input: Write a python function to sort a list of Employee objects by their salary in ascending order.
  assert sort_employees_by_salary([Employee(1, 'Alice', 50000), Employee(2, 'Bob', 40000), Employee(3, 'Charlie', 60000)]) == [Employee(2, 'Bob', 40000), Employee(1, 'Alice', 50000), Employee(3, 'Charlie', 60000)]
  assert sort_employees_by_salary([Employee(1, 'Alice', 70000), Employee(2, 'Bob', 70000), Employee(3, 'Charlie', 60000)]) == [Employee(3, 'Charlie', 60000), Employee(1, 'Alice', 70000), Employee(2, 'Bob', 70000)]
  assert sort_employees_by_salary([]) == []
output: def sort_employees_by_salary(employees):
      return sorted(employees, key=lambda emp: emp.salary)