[17.0][FIX]he_timesheet_sheet: don't copy sheet when creating a new line via button

[PicchiSeba]

The problem described here https://github.com/OCA/timesheet/pull/754 also happens in 17.0, this PR should fix it

[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.