input: Write a function to convert a given SQL statement into a Python Pandas DataFrame. The function should return the DataFrame generated from the SQL query. If the input SQL statement is invalid or cannot be executed, the function should return None.
  
  assert convert_sql_to_df("SELECT * FROM book;") is not None
  assert convert_sql_to_df("SELECT title, author FROM book;") is not None
  assert convert_sql_to_df("INVALID SQL STATEMENT") is None
  assert convert_sql_to_df("") is None
  assert convert_sql_to_df(None) is None
output: 
  def convert_sql_to_df(sql_statement):
      if not sql_statement:
          return None
      try:
          import pandas as pd
          import sqlite3
          conn = sqlite3.connect(':memory:')
          df = pd.read_sql_query(sql_statement, conn)
          conn.close()
          return df
      except:
          return None
  