[web] Empty password caused "Unknown error" during login

Summary: Solving this: https://linear.app/comm/issue/ENG-2655/[web]-unknown-error-when-password-is-empty-during-login

Test Plan:
Tested correct and incorrect passwords.

Reviewers: tomek, inka, ashoat

Reviewed By: tomek, ashoat

Subscribers: ashoat, atul, inka, tomek

Differential Revision: https://phab.comm.dev/D6182