Avoid endless loading cycle when inputting empty password

[fallenwood]

• I have disclosed use of any AI generated code in my commit messages.
  • If you are using an LLM, and do not fully understand the changes it is making to the code base, do not create a PR.
  • In our experience, AI generated code often results in overly complex code that lacks enough context for a proper fix or feature inclusion. This results in considerably longer code reviews. Due to this, AI authored or partially authored PRs may be closed without comment.
• I understand these changes in full and will be able to respond to review comments.
• My change is accurately described in the commit message.
• My contribution is tested and working as described.
• I have read the Developer Certificate of Origin and certify my contribution under its conditions.

Environment: Fedora 44, amd64

Current Behavior: It enters endless loading cycle if the input password is empty, also see #407

New Behavior: It aligns with PAM, if the password is empty,

1. It shows another prompt for password from PAM, so the greeter does nothing to wait for user to input password again
2. If the password is empty on the next try, it show incorrent password prompt

LLM Usage:

1. Github Copilot with GPT-5.4
2. Github Copilot with GPT-5.6-sol

[mmstick]

Shouldn't this logic be in cosmic-greeter instead of the example? Is this necessary for the example to check if a password is empty?

[fallenwood]

It's used to simulate the real case, because it has different behavior on my fedora devbox when sending empty password for the 1st time and later

[mmstick]

PR requires to be rebased

[fallenwood]

PR requires to be rebased

Thanks, I found the issue got regression after resolving conflicts, I'll take another look on it

[fallenwood]

Done rebasing