input: Write a function to convert a given SQL statement into a Python Pandas DataFrame. The function should take as input the SQL statement and a database connection object, and return the resulting DataFrame. If the SQL statement is invalid or the connection object is None, the function should return None.
  
  assert convert_sql_to_df("SELECT * FROM book;", conn).equals(pd.read_sql_query("SELECT * FROM book;", conn)) == True
  assert convert_sql_to_df("SELECT title, author FROM book WHERE year > 2000;", conn).equals(pd.read_sql_query("SELECT title, author FROM book WHERE year > 2000;", conn)) == True
  assert convert_sql_to_df("", conn) == None
  assert convert_sql_to_df("SELECT * FROM book;", None) == None
output: def convert_sql_to_df(sql_statement, conn):
      if not sql_statement or conn is None:
          return None
      try:
          return pd.read_sql_query(sql_statement, conn)
      except:
          return None