[FIX] project: add missing stage when import tasks

Before this commit:
Let's consider project P1 having no stages.
Let's consider project P2 having Stage 1, Stage 2
When importing tasks via csv file:
- Task 1 for project P1 and Stage 1
- Task 2 for project P1 and Stage 2

Stage 1 and Stage 2 are still assigned only to project P1.

After this commit:
Stage 1 and Stage 2 are now assigned to P1 and P2.

Task-2996393

closes odoo/odoo#103462

X-original-commit: 73de38e0
Signed-off-by: Xavier <xbo@odoo.com>