[FIX] website_profile: show + only when gained karma is positive

Before this commit, in the website profile frontend, if we filter the users
using 'This week' or 'This month' filters we always get a '+' displayed before
the karma gain.

If user gained negative karma, lets say -500, currently it displays as follows
--> +-500 , instead of -500.

After this commit, if the gained karma during the period is it will show
-500 instead of +-500.

closes odoo/odoo#114506

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