input: Write a function to convert a given SQL statement into a Python Pandas DataFrame. The function should take a SQL query string and a connection object to a database as inputs, and return a Pandas DataFrame containing the results of the query. If the SQL query is invalid or the connection fails, the function should return None.
  assert convert_sql_to_df("SELECT * FROM book;", conn).equals(pd.read_sql_query("SELECT * FROM book;", conn))
  assert convert_sql_to_df("INVALID SQL QUERY", conn)==None
output: def convert_sql_to_df(sql_query, conn):
      try:
          import pandas as pd
          return pd.read_sql_query(sql_query, conn)
      except:
          return None