I don't think you'd want a password box to be sending itself to a client. Since you don't want to show the value anyway, let it be blank.
Test the value on the server. If the admin types in a new password, set it on the user. If the value is blank, leave the current password as is.
If you want to allow blank passwords, you'll need something different. I'd suggest an explicit "Password Reset" page.
Actually, I'd suggest that regardless.