[FW][FIX] project: propagate internal subtypes to tasks

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

closes odoo/odoo#62495

X-original-commit: odoo/odoo@2ad320e9050160725601b82fa6239c94aad93d20
X-original-pr: odoo/odoo#47336
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>