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 DataFrame.
  def sql_to_dataframe(sql_query, conn):
      import pandas as pd
      df = pd.read_sql_query(sql_query, conn)
      return df
  
  assert sql_to_dataframe("SELECT * FROM book;", conn).shape[0] > 0
  assert sql_to_dataframe("SELECT * FROM author;", conn).shape[0] > 0
  assert sql_to_dataframe("SELECT * FROM customer;", conn).shape[1] > 0
  assert sql_to_dataframe("SELECT * FROM order;", conn).empty == False
  assert sql_to_dataframe("SELECT * FROM invalid_table;", conn).empty == True
output: def sql_to_dataframe(sql_query, conn):
      import pandas as pd
      df = pd.read_sql_query(sql_query, conn)
      return df