input: Write a function to convert a given SQL statement into a Python Pandas DataFrame. The SQL statement is provided as a string and the function should use the `pandas.read_sql_query` method to execute the SQL query. The function should return the resulting DataFrame. If the SQL query is invalid or the connection is not successful, the function should return `None`.
  
  def sql_to_dataframe(sql_statement, conn):
      import pandas as pd
      try:
          df = pd.read_sql_query(sql_statement, conn)
          return df
      except:
          return None
  
  assert sql_to_dataframe("SELECT * FROM book;", conn) is not None
  assert sql_to_dataframe("INVALID SQL QUERY", conn) == None
output: def sql_to_dataframe(sql_statement, conn):
      import pandas as pd
      try:
          df = pd.read_sql_query(sql_statement, conn)
          return df
      except:
          return None