[FIX] project: propagate internal subtypes to tasks

Manual backport of commit done in 13.0 odoo/odoo@227c8fb53265de03b202133c4e8012b8bcce01b4

before this commit:

while changing subtypes of the project it was only
propagating those subtypes to the task, whose parent_id is set.
internal subtypes are not propagating in the task of a project.

So, filter internal subtypes and concat them with the subtypes which
have parent_id.

after this commit:
internal subtypes will also propagate to their tasks

LINKS

Task ID-2205643
Task ID-2241688
COM PR #56775

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>