Typings for `validateScope` don't correctly reflect that `scope` arg can be undefined

[dhensby]

Specify your setup

• Operating System: MacOS
• Node version: 18
• npm version: 8
• version of @node-oauth/oauth2-server: 5.0.0
• which OAuth2 workflow: client_credentials
• at which workflow step does the error occur: all

Describe the bug

Typings for validateScope don't correctly reflect that scope arg can be undefined

To Reproduce

Steps to reproduce the behavior:

1. Receive an HTTP request with no scope parameter
2. Note that scope parameter to model.validateScope() method is `undefined

Expected behavior

The typings correctly reflect that the scope argument is optional.

Screenshots

n/a

Additional context

The scope util that is responsible for parsing the scopes from a request (https://github.com/node-oauth/node-oauth2-server/blob/v5.0.0/lib/utils/scope-util.js#L10-L12) returns undefined when no scope parameter is present and this return value is then passed to the validateScope() method.