[15.0][FIX] change role_line_ids default value to override create

[PaoloYam]

Populating role_line_ids this way doesn't work in this case. The previous method indeed return the right value but at creation, role_line_ids is empty in vals resulting of a new user having no role attributed even though the default_user has one.

Feature already has test. Test added for coverage.

[OCA-git-bot]

Hi @novawish, @sebalix, @jcdrubay, some modules you are maintaining are being modified, check this out!

[OCA-git-bot]

This PR has the approved label and has been created more than 5 days ago. It should therefore be ready to merge by a maintainer (or a PSC member if the concerned addon has no declared maintainer). 🤖

[github-actions[bot]]

There hasn't been any activity on this pull request in the past 4 months, so it has been marked as stale and it will be closed automatically if no further activity occurs in the next 30 days. If you want this PR to never become stale, please ask a PSC member to apply the "no stale" label.