input: Write a function to convert a given SQL statement into a Python Pandas DataFrame. The function should take the SQL statement as a string and the database connection object as parameters. The function should return the DataFrame resulting from executing the SQL query.
  
  assert convert_sql_to_df("SELECT * FROM book;", conn).equals(pd.read_sql_query("SELECT * FROM book;", conn))
  assert convert_sql_to_df("SELECT title, author FROM book;", conn).equals(pd.read_sql_query("SELECT title, author FROM book;", conn))
  assert convert_sql_to_df("SELECT * FROM customer;", conn).equals(pd.read_sql_query("SELECT * FROM customer;", conn))
  assert convert_sql_to_df("", conn) is None
  
  def convert_sql_to_df(sql_query, conn):
      if sql_query:
          return pd.read_sql_query(sql_query, conn)
      return None
output: def convert_sql_to_df(sql_query, conn):
      if sql_query:
          return pd.read_sql_query(sql_query, conn)
      return None