Password sync sends blank password and login ignores configured xWeb URL

Problem/Motivation
1. Saving the user profile form without entering a new password sends a blank password to netFORUM. Users without a netFORUM account get a SOAP error and can't save their profile.
2. Login, SSO token and cst_key lookups always use https://netforum.avectra.com/xWeb/Signon.asmx?WSDL and ignore the xWeb WSDL URL in the module settings.
3. If netFORUM is unreachable or not configured, an uncaught SoapFault breaks the login form for all users, including admins.

Proposed resolution
- Only sync the password when a new one is entered, and only for users with a netFORUM account.
- Build the Signon.asmx URL from the configured xWeb WSDL URL, falling back to the old default.
- Skip netFORUM when the module isn't enabled, and log SOAP errors and fall back to the normal Drupal login.