input: Write a function to predict the next day's temperature using exponential smoothing.
  assert predict_temperature([21, 22, 20]) == 21.25
  assert predict_temperature([30, 32, 29, 35, 33]) == 34.125
  assert predict_temperature([15, 14, 13, 12, 11]) == 12.5
output: def predict_temperature(temperatures, alpha=0.5):
      if not temperatures:
          return None
      
      prediction = temperatures[0]
      for temp in temperatures[1:]:
          prediction = alpha * temp + (1 - alpha) * prediction
      
      return prediction