input: Write a python function to replace empty strings in a list with the previous non-empty string.
  assert replace_empty_strings(['a', '', 'b', '', 'c']) == ['a', 'a', 'b', 'b', 'c']
  assert replace_empty_strings(['', 'x', '', 'y', 'z']) == ['', 'x', 'x', 'y', 'z']
  assert replace_empty_strings(['x', '', '', 'y', '', 'z', '']) == ['x', 'x', 'x', 'y', 'y', 'z', 'z']
output: def replace_empty_strings(lst):
      for i in range(1, len(lst)):
          if lst[i] == '':
              lst[i] = lst[i-1]
      return lst