input: Write a function to create an HTML form with a submit button to send information to a specified Python program. The form should have a 'post' method and the action should point to the given Python program. If the action is not provided, the form should not have an action attribute.
  assert create_html_form("action.py")=='<form action="action.py" method="post"><input type="submit" value="Submit"></form>'
  assert create_html_form("")=='<form method="post"><input type="submit" value="Submit"></form>'
  assert create_html_form(None)=='<form method="post"><input type="submit" value="Submit"></form>'
output: def create_html_form(action):
      if action:
          return f'<form action="{action}" method="post"><input type="submit" value="Submit"></form>'
      else:
          return '<form method="post"><input type="submit" value="Submit"></form>'