input: Write a python function to extract product names from a list of dictionaries and handle cases where the product name is missing.
  assert extract_product_names([{'product_name': 'Laptop'}, {'product_name': 'Phone'}, {'product_name': None}]) == ['Laptop', 'Phone']
  assert extract_product_names([{'product_name': 'Shoes'}, {'product_name': 'Watch'}, {'product_name': 'Bag'}]) == ['Shoes', 'Watch', 'Bag']
  assert extract_product_names([{'product_name': None}, {'product_name': 'Tablet'}, {'product_name': None}]) == ['Tablet']
output: def extract_product_names(products):
      return [product['product_name'] for product in products if product['product_name']]