input: Write a function to convert a given SQL statement into a Python Pandas DataFrame. The SQL statement should be passed as a string, and the function should return a Pandas DataFrame. The connection object to the SQL database should be provided as an argument to the function.
  assert convert_sql_to_df("SELECT * FROM book;", conn).shape[0] > 0
  assert convert_sql_to_df("SELECT * FROM book;", conn).columns.tolist() == ['title', 'author', 'year']
  assert convert_sql_to_df("", conn) == None
  assert convert_sql_to_df("SELECT title, author FROM book;", conn).shape[1] == 2
output: def convert_sql_to_df(sql_query, conn):
      if not sql_query:
          return None
      return pd.read_sql(sql_query, conn)