PAM Auth with password encryption turned off causes front-end validation of username and password to not run

[kfeinauer]

When password encryption is turned off, front-end validation for the login page is disabled. This allows users to submit empty usernames and/or passwords, causing the back-end PAM helper process to hang.

We should ensure that validation runs regardless of the password encryption, and also make the back-end services more resilient to invalid input.

To reproduce:

Make sure you're logged out of RStudio
Disable password auth (via setting or by load balancing)
Log in to RStudio with an empty username or password
Observe that the page hangs

RSP 1.1.374 (all platforms).