input: Write a function to convert a given SQL statement into a Python Pandas DataFrame. The function should accept a SQL statement and a database connection as input, and return a Pandas DataFrame. If the given SQL statement is empty, the function should return None.
  
  assert to_pandas_dataframe("SELECT * FROM book;", conn).equals(pd.read_sql_query("SELECT * FROM book;", conn))
  assert to_pandas_dataframe("", None)==None
  assert to_pandas_dataframe("SELECT title, author FROM book;", conn).equals(pd.read_sql_query("SELECT title, author FROM book;", conn))
  assert to_pandas_dataframe("SELECT * FROM non_existing_table;", conn)==None
  assert to_pandas_dataframe("SELECT * FROM book WHERE genre='Fiction';", conn).equals(pd.read_sql_query("SELECT * FROM book WHERE genre='Fiction';", conn))
output: def to_pandas_dataframe(sql_statement, db_connection):
      if not sql_statement:
          return None
      try:
          return pd.read_sql_query(sql_statement, db_connection)
      except:
          return None