Before this commit, the date_last_stage_update field in fill everytime at task creation. This does not make sense if ther is no project linked to the task or if there is no stages associated to the project/task. This commit populates the date_last_stage_update field only when needed. So, we can now have newly created task without date_last_stage_update filled. closes odoo/odoo#31999 Signed-off-by: Jérome Maes (jem) <jem@openerp.com>