input: Write a function to return the most common fruit name from a given list of fruit names. 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'])=='Apple'
  assert most_common_fruit(['Grape', 'Grape', 'Grape', 'Apple', 'Apple'])=='Grape'
  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)