input: Write a function to convert a given SQL statement into a Python Pandas DataFrame. The function should take the SQL query as a string and a database connection object as parameters and return the resulting DataFrame.
  assert convert_sql_to_df("SELECT * FROM book;", conn).equals(pd.DataFrame({'id': [1, 2], 'title': ['Book1', 'Book2']})) == True
  assert convert_sql_to_df("SELECT id FROM book;", conn).equals(pd.DataFrame({'id': [1, 2]})) == True
  assert convert_sql_to_df("SELECT title FROM book;", conn).equals(pd.DataFrame({'title': ['Book1', 'Book2']})) == True
  assert convert_sql_to_df("SELECT * FROM book WHERE id > 1;", conn).equals(pd.DataFrame({'id': [2], 'title': ['Book2']})) == True
  assert convert_sql_to_df("", conn) is None
output: def convert_sql_to_df(sql_query, conn):
      if not sql_query:
          return None
      return pd.read_sql(sql_query, conn)