input: Write a python function to merge a list of lists into a single list.
  assert merge_lists([[1, 2, 3], [4, 5], [6]]) == [1, 2, 3, 4, 5, 6]
  assert merge_lists([[], [1, 2], [3], []]) == [1, 2, 3]
  assert merge_lists([['a', 'b'], ['c', 'd'], ['e']]) == ['a', 'b', 'c', 'd', 'e']
output: def merge_lists(list_of_lists):
      merged_list = []
      for sublist in list_of_lists:
          merged_list.extend(sublist)
      return merged_list