input: Write a function that takes a list containing fruit names and returns the most common fruit name. If there are multiple fruits with the same maximum frequency, return any one of them. If the list is empty, return None.
  assert most_common_fruit(['Apple', 'Orange', 'Apple', 'Pineapple', 'Grape', 'Apple'])=='Apple'
  assert most_common_fruit(['Orange', 'Apple', 'Pineapple', 'Pineapple', 'Grape'])=='Pineapple'
  assert most_common_fruit([])==None
output: def most_common_fruit(fruits):
      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
      return max(fruit_count, key=fruit_count.get)