input: Write a function to merge two sorted lists into a single sorted list. If the lists are empty, the function should return an empty list.
  assert merge_sorted_lists([2, 4, 6], [3, 5, 7])==[2, 3, 4, 5, 6, 7]
  assert merge_sorted_lists([], [1, 2, 3])==[1, 2, 3]
  assert merge_sorted_lists([4, 5], [])==[4, 5]
  assert merge_sorted_lists([1, 3, 5], [2, 4, 6])==[1, 2, 3, 4, 5, 6]
  assert merge_sorted_lists([], [])==[]
output: def merge_sorted_lists(list1, list2):
      result = []
      i, j = 0, 0
      while i < len(list1) and j < len(list2):
          if list1[i] < list2[j]:
              result.append(list1[i])
              i += 1
          else:
              result.append(list2[j])
              j += 1
      result.extend(list1[i:])
      result.extend(list2[j:])
      return result