Fix form submission error when 'Use technician group on ticket creation' is enabled

[Lainow]

Checklist before requesting a review

Please delete options that are not relevant.

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

• It fixes !46738
• Here is a brief description of what this PR does

When the core auto-assigns the technician (from the ITIL category or the item), _users_id_assign is a scalar ID, not an array. assignUserGroup() now handles both.

Screenshots (if appropriate):