input: Write a function to convert a given SQL statement into a Python Pandas DataFrame. The function should take the SQL statement as a string and a database connection object as input, and return a Pandas DataFrame.
  
  def sql_to_dataframe(sql_query, conn):
      import pandas as pd
      return pd.read_sql_query(sql_query, conn)
  
  assert sql_to_dataframe("SELECT * FROM book;", conn).equals(pd.read_sql_query("SELECT * FROM book;", conn)), "Test case 1 failed"
  assert sql_to_dataframe("SELECT title, author FROM book;", conn).equals(pd.read_sql_query("SELECT title, author FROM book;", conn)), "Test case 2 failed"
  assert sql_to_dataframe("SELECT * FROM author;", conn).equals(pd.read_sql_query("SELECT * FROM author;", conn)), "Test case 3 failed"
  assert sql_to_dataframe("SELECT * FROM non_existent_table;", conn).empty, "Test case 4 failed"
  assert sql_to_dataframe("SELECT id FROM book WHERE id < 10;", conn).equals(pd.read_sql_query("SELECT id FROM book WHERE id < 10;", conn)), "Test case 5 failed"
  
  # conn should be a valid database connection object
output: def sql_to_dataframe(sql_query, conn):
      import pandas as pd
      return pd.read_sql_query(sql_query, conn)