[Monitoring] HTTP Exporter should require the password when username is set

[elasticmachine]

Original comment by @pickypg:

Currently we allow the username to be set without the password in the HTTP Exporter. However, this results in a Basic Auth request like elastic:null (assuming elastic was the username), which is definitely not what the other side is going to expect for a blank password.

This was allowed in the first place because the Apache client accepts null, but since it literally passes null, that seems like a worthless thing to support.

[danhermann]

This should be resolved by #47821