next() is optional in middleware in TypeScript

[seratch]

Description

For all middleware functions, next argument exists for sure. However, the types in bolt-js has not been representing that. As the argument is always optional, developers using TypeScript have to call the method with !.

app.use(async ({ next }) => {
  await next!();
});

The type definition: https://github.com/slackapi/bolt-js/blob/%40slack/bolt%403.6.0/src/types/middleware.ts#L24-L25
Examples: https://github.com/slackapi/bolt-js/blob/%40slack/bolt%403.6.0/src/App.spec.ts#L428-L431

What type of issue is this? (place an x in one of the [ ])

• bug
• enhancement (feature request)
• question
• documentation related
• example code related
• testing related
• discussion

Requirements (place an x in each of the [ ])

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've searched for any related issues and avoided creating a duplicate issue.