[FIX] survey: fix matrix prepare answer method

The matrix data structure awaited by the server is the following :
{
    question_id: {
        'row_id_1': ['col_id1', ..., 'col_idn'],
        'row_id_n': ['col_id1', ..., 'col_idn'],
        'comment': 'This is a comment',
    }
}

Before this commit, the comment was encapsulated into a dict, into an array,
with an incorrect key :
    -> 'questionId_comment': [{'comment': 'This is a comment'}]

This was leading to 2 errors :
- A traceback when nothing is selected in the matrix options
- Comment is never saved.

After this commit, the comment is correctly given to and extracted by the
server. So the comment is saved and the traceback is avoided.

Task ID: 2152223

closes odoo/odoo#43086

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>
Co-authored-by: Aurélien Warnon <awa@odoo.com>