input: Write a function to iterate over a list of numbers and print only the even numbers.
  assert for_loop_even_numbers([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])==2, 4, 6, 8, 10
  assert for_loop_even_numbers([2, 4, 6, 8, 10])==2, 4, 6, 8, 10
  assert for_loop_even_numbers([1, 3, 5, 7, 9])==None
  assert for_loop_even_numbers([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12])==2, 4, 6, 8, 10, 12
  assert for_loop_even_numbers([])==None
output: def for_loop_even_numbers(numbers):
      even_numbers = [num for num in numbers if num % 2 == 0]
      if even_numbers:
          return ', '.join(map(str, even_numbers))
      else:
          return None