input: Write a function to determine the most common fruit name given a list of fruit names. The function should return the fruit name that appears the most in the list. If there is a tie, return any one of the most common fruits. If the list is empty, return None.
  assert most_common_fruit(['Apple', 'Orange', 'Apple', 'Pineapple', 'Grape', 'Apple'])=='Apple'
  assert most_common_fruit(['Apple', 'Orange', 'Orange', 'Apple', 'Grape', 'Orange'])=='Orange'
  assert most_common_fruit(['Apple', 'Orange', 'Pineapple', 'Grape'])=='Apple'
  assert most_common_fruit([])==None
output: if not fruits:
      return None
  
  fruit_count = {}
  for fruit in fruits:
      if fruit in fruit_count:
          fruit_count[fruit] += 1
      else:
          fruit_count[fruit] = 1
  
  most_common = max(fruit_count, key=fruit_count.get)
  return most_common