input: Write a function to convert the given temperature from Celsius to Fahrenheit. The function should take a Celsius temperature as input and return the equivalent temperature in Fahrenheit. If the input temperature is empty or None, the function should return None.
  assert celsius_to_fahrenheit(0) == 32
  assert celsius_to_fahrenheit(100) == 212
  assert celsius_to_fahrenheit(-40) == -40
  assert celsius_to_fahrenheit(None) == None
output: def celsius_to_fahrenheit(celsius):
      if celsius is None:
          return None
      return (celsius * 9/5) + 32