input: Write a python function to repeat the last character of a string as many times as the length of the string.
  assert repeat_last_character('Hello') == 'ooooo'
  assert repeat_last_character('Python') == 'nnnnnn'
  assert repeat_last_character('a') == 'a'
  assert repeat_last_character('') == ''
output: def repeat_last_character(s):
      if not s:
          return ''
      return s[-1] * len(s)