input: Write a function to convert a given SQL statement into a Python Pandas DataFrame. The function should take a SQL statement string as input and return a Pandas DataFrame. If the SQL statement is invalid or there is no data to return, the function should return an empty DataFrame.
  assert convert_sql_to_df("SELECT * FROM book;").equals(pd.read_sql_query("SELECT * FROM book;", conn))==True
  assert convert_sql_to_df("SELECT * FROM invalid_table;").empty==True
  assert convert_sql_to_df("INVALID SQL STATEMENT").empty==True
  assert convert_sql_to_df("").empty==True
  assert convert_sql_to_df(None).empty==True
output: def convert_sql_to_df(sql_statement):
      import pandas as pd
      try:
          if sql_statement is None or sql_statement.strip() == "":
              return pd.DataFrame()
          return pd.read_sql_query(sql_statement, conn)
      except:
          return pd.DataFrame()