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 input is not a valid SQL statement, the function should return an empty DataFrame.
  assert convert_sql_to_df("SELECT * FROM book;") == pd.read_sql_query("SELECT * FROM book;", conn)
  assert convert_sql_to_df("SELECT author, title FROM book WHERE year > 2000;") == pd.read_sql_query("SELECT author, title FROM book WHERE year > 2000;", conn)
  assert convert_sql_to_df("Invalid SQL Statement") == pd.DataFrame()
  assert convert_sql_to_df("") == pd.DataFrame()
output: def convert_sql_to_df(sql_statement):
      try:
          import pandas as pd
          from sqlalchemy import create_engine
          engine = create_engine('sqlite:///:memory:')
          return pd.read_sql_query(sql_statement, engine)
      except:
          return pd.DataFrame()