Fix empty password submitting failure on admin editing user page

What does this MR do and why?

When the admin want to edit a user, but not changing user's password, admin can submit the form without filling password field, if password complexity is turned on, the form submitting is blocked by frontend.

This MR is to make the submitting without password value can work again when admin does not intend to change user's password.

Screenshots or screen recordings

How to set up and validate locally

1. Set up gitlab in EE premium plan at least
2. go to http://{YOUR_HOST}/admin/application_settings/general and expand Sign up restrictions and turn on password complexity
3. go to http://127.0.0.1:3000/admin/users edit any user
4. submit the form with an empty value of password

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

@daveliu @orozot @JeremyWuuuuu