input: Write a function to convert a given temperature value from Fahrenheit to Celsius. The function should return the temperature in Celsius rounded to two decimal places. If the input temperature is empty, the function should return None.
  assert fahrenheit_to_celsius(32)==0.0
  assert fahrenheit_to_celsius(212)==100.0
  assert fahrenheit_to_celsius(98.6)==37.0
  assert fahrenheit_to_celsius(-40)==-40.0
  assert fahrenheit_to_celsius(None)==None
output: def fahrenheit_to_celsius(temp):
      if temp is None:
          return None
      return round((temp - 32) * 5 / 9, 2)