Handle missing SSH mirror host URLs

What does this MR do and why?

Sentry error: https://new-sentry.gitlab.net/organizations/gitlab/issues/4011377

Problem

Requests without an ssh_url parameter made SshHostKey#normalize_url call inferred_port on nil, resulting in an unhandled 500 response.

Solution

• Reject blank URLs with ArgumentError, "Invalid URL".
• Reuse the controller's error handling to return a 400 response.
• Add model and controller regression specs.

References

NoMethodError: undefined method `inferred_port' for nil (NoMethodError)

    url.port = url.inferred_port
                  ^^^^^^^^^^^^^^
  from app/models/ssh_host_key.rb:158:in `normalize_url'
  from app/models/ssh_host_key.rb:61:in `initialize'
  from app/controllers/projects/mirrors_controller.rb:61:in `new'
  from app/controllers/projects/mirrors_controller.rb:61:in `ssh_host_keys'

How to set up and validate locally

1. Run bin/rspec spec/models/ssh_host_key_spec.rb spec/controllers/projects/mirrors_controller_spec.rb.
2. Run RuboCop on the three changed files.

MR acceptance checklist

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