input: Write a function to convert ratings in a list of strings to a float.
  def str_to_float(ratings):
      return [float(x) for x in ratings]
  
  assert str_to_float(['4.5', '3.0', '2.5', '4.0'])==[4.5, 3.0, 2.5, 4.0]
  assert str_to_float(['0.0', '1.5', '5.0'])==[0.0, 1.5, 5.0]
  assert str_to_float([''])==[0.0]
  assert str_to_float([])==[]
output: def str_to_float(ratings):
      return [float(x) if x else 0.0 for x in ratings]