input: Write a python function to fetch all rows from a table using cx_Oracle.
  assert fetch_all_rows('<username>', '<password>', '<database_url>', 'my_table') == [(1, 'Alice'), (2, 'Bob'), (3, 'Charlie')]
  assert fetch_all_rows('<username>', '<password>', '<database_url>', 'empty_table') == []
output: def fetch_all_rows(username, password, database_url, table_name):
      import cx_Oracle
  
      dsn = cx_Oracle.makedsn(database_url, 1521)
      connection = cx_Oracle.connect(user=username, password=password, dsn=dsn)
      
      cursor = connection.cursor()
      query = f"SELECT * FROM {table_name}"
      cursor.execute(query)
      
      rows = cursor.fetchall()
      
      cursor.close()
      connection.close()
      
      return rows