In this challenge, you'll be working with a DataFrame that contains data about artworks, and it contains many missing values. Your task is to create a variable called na_sum that contains the total number of missing values in the DataFrame. When that's completed, print out your answer! Hint: The code given below will give you the number of missing (NaN) values for the Name column in the DataFrame. How would you edit the code to get the missing values for every column in the DataFrame? Extra hint: You'll be returning a single number which is the final sum() of everything. df['Name'].isnull().sum()