input: Write a function to convert a given SQL statement into a Python Pandas DataFrame. The function should read the SQL query from a string and return the 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 == (5, 4)
  assert sql_to_dataframe("SELECT * FROM author;", conn).shape == (3, 2)
  assert sql_to_dataframe("SELECT * FROM book WHERE id > 2;", conn).shape == (3, 4)
  assert sql_to_dataframe("", conn) == pd.DataFrame()
  assert sql_to_dataframe("SELECT title FROM book;", conn).shape == (5, 1)
  assert sql_to_dataframe("SELECT title FROM book WHERE id > 2;", conn).shape == (3, 1)
  assert sql_to_dataframe("SELECT title, author FROM book;", conn).shape == (5, 2)
  assert sql_to_dataframe("SELECT title, author FROM book WHERE id > 2;", conn).shape == (3, 2)
output: def sql_to_dataframe(sql_query, conn):
      import pandas as pd
      if sql_query:
          return pd.read_sql_query(sql_query, conn)
      else:
          return pd.DataFrame()