input: Write a function to convert the given temperature from Celsius to Fahrenheit. The function should take the temperature in Celsius as input and return the temperature in Fahrenheit. If the input is empty then return None.
  
  def celsius_to_fahrenheit(celsius):
      if celsius is None:
          return None
      fahrenheit = (celsius * 9/5) + 32
      return fahrenheit
  
  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
      fahrenheit = (celsius * 9/5) + 32
      return fahrenheit