Do not propagate none password to http backend (backport #13408)

[mergify]

Proposed Changes

Do not propagate password if not provided. Internally, when a connection authenticates with a client certificate, the password credentials is set to none. When RabbitMQ delegates the authentication to a http backend, RabbitMQ should not send a password whose value is the Erlang term none.

Types of Changes

What types of changes does your code introduce to this project?
Put an x in the boxes that apply

• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

---

This is an automatic backport of pull request #13408 done by [Mergify](https://mergify.com).