[FIX] gamification: prevent division by zero in percentage completeness computation

We are initializing over the limit to make sure we will compute it at least once.
If the target goal is set to 0, we are facing a division by zero error when
displaying the gamification goal just after initialization.

closes odoo/odoo#40729

X-original-commit: 24546e7c4cac61efea8d3f9e558518a55ef9acd2
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>