input: Write a function to convert a given SQL statement into a Python Pandas DataFrame. The function should take a SQL query as a string and a SQL connection object (conn) as arguments, and return the resulting DataFrame. If the input SQL query is invalid or the connection is not properly established, the function should return an empty DataFrame.
  
  assert convert_sql_to_df("SELECT * FROM book;", conn).equals(pd.read_sql_query("SELECT * FROM book;", conn))==True
  assert convert_sql_to_df("", conn).empty==True
  assert convert_sql_to_df("SELECT * FROM book;", None).empty==True
output: def convert_sql_to_df(sql_query, conn):
      if not sql_query or conn is None:
          return pd.DataFrame()
      try:
          return pd.read_sql_query(sql_query, conn)
      except:
          return pd.DataFrame()