input: Write a function to create an HTML form with a submit button to send information to a Python program. If the input is empty, return an empty string. If the input is valid, return the HTML form as shown in the example.
  assert create_form("")==""
  assert create_form("action.py")=='<form action="action.py" method="post"><input type="submit" value="Submit"></form>'
output: def create_form(action):
      if not action:
          return ""
      return f'<form action="{action}" method="post"><input type="submit" value="Submit"></form>'