Registry of type npm-registry should not require a username/password or token

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Feature description

Dependabot should be able to use a public registry without requiring a username/password or token. Currently, Dependabot gives this [error](https://github.com/GMZwinge/test-github/network/updates):

```
Dependabot encountered the following error when parsing your .github/dependabot.yml:

The property '#/registries/npm-npmjs/' of type object did not match one or more of the required schemas
Please update the config file to conform with Dependabot's specification.
```

with a [dependabot.yml](https://github.com/GMZwinge/test-github/blob/main/.github/dependabot.yml) like this:

```
version: 2
registries:
npm-npmjs:
type: npm-registry
url: https://registry.npmjs.org
updates:
- package-ecosystem: npm
directory: /testDependabot
schedule:
interval: monthly
groups:
test:
dependency-type: development
registries:
- npm-npmjs
```

Adding empty username/password or token entry doesn't avoid the problem.

This could apply to other type of public registry (maven, etc), and possibly to #3646.

Start with the reported .github/dependabot.yml and the schema error for the npm-npmjs registry. Trace the registry configuration validation that rejects a public npm registry without credentials, then check how the issue's possible Maven and other public-registry cases affect the scope. Done means the shown configuration is accepted without username, password, or token, with coverage for the agreed registry types.

Written by the indexing model from the issue text.