input: Write a function to convert a given SQL statement into a Python Pandas DataFrame. The function should take a SQL statement as input and return a Pandas DataFrame. If the SQL statement is invalid or the connection to the database fails, the function should return None.
  assert convert_sql_to_df("SELECT * FROM book;") is not None
  assert convert_sql_to_df("INVALID SQL STATEMENT;") == None
  assert convert_sql_to_df("") == None
  assert convert_sql_to_df("SELECT * FROM author;") is not None
  assert convert_sql_to_df("SELECT title FROM book WHERE author = 'Mark Twain';") is not None
output: 
  import pandas as pd
  import sqlite3
  
  def convert_sql_to_df(sql_statement):
      try:
          conn = sqlite3.connect(':memory:')
          df = pd.read_sql_query(sql_statement, conn)
          conn.close()
          return df
      except Exception:
          return None
  