[FIX] survey: missing questions without section in survey

Since commit https://github.com/odoo/odoo/commit/dba478332681dd21a39c4a183c002b1cf6e046d2

, a survey can have questions with no section
associated, which means 'page_ids' field will be empty.

This prevents to display some questions of a survey, as we are
still expecting a structure page/questions. To solve it, we first
append questions with no sections, then all other questions.

closes odoo/odoo#39113

X-original-commit: 149657cf
Signed-off-by: Alex Tuyls <alt-odoo@users.noreply.github.com>