ws: Redo check for empty user name.

Parsing the auth input now allows a empty user name.  Empty user names
will lead to authentication failures later on, which is what we want.

Previously, a empty user name was a syntax error.  Recently, all
syntax errors were reported as authentication failures.

This reverts the backend part of "3fd6344 shell: Fix bad response
when login with empty user name" and "a9e6943 ws: Update test cases
for previous commit".
I personally wouldn't trust passing an empty user name through the authentication stack. It seems too risky, especially now that we use setuid cockpit-session, we need to make sure that we have multiple cross-checks and fail-safes.