Pinging @wouterj

This PR is related to #53851 's Context.

A person going through Symfony docs for the first time wanted to create their own LoginFormType as a next step in their learning Symfony journey and noticed that you can submit empty username/password with form login.

They wanted to disallow this and tried to add validation. To validate a login form is not so straight forward as it either needs to be done with a custom authenticator (complex validation) or user provider if the data checks are simple.

Following comments:

#53851 (comment)

Given the broken high-deps build, I wonder if this shouldn't even be done with a deprecation notice before making it throw in 8.0?

#53851 (comment)

These are 3 tests submitting an empty login form to trigger a CSRF token error. This new condition now takes precedence, meaning it returns the wrong error.
I don't think that is something we have to worry about (in both situations, login errors), it rather reveals a bad test in our codebase. I can't think of a use-case that would result in success and will become a failure after this merge.

#53851 (comment)

I think we need consensus on whether we find this a hard BC break that deserves a smooth upgrade path, but the test need to be fixed whatever the conclusion