Before this commit, when computing the result graph data in the survey finished
page, a user answer for a question with no correct answer was counted twice,
once as correct, once as incorrect, as :
correct_answer == question_correct_answer == none
After this commit, we count an answer as correct only if question has a correct
answer.
Task ID: 2176630
closesodoo/odoo#54755
X-original-commit: 74a00111908556bce10ea3a5e748f5a5c81f6a40
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>