Bug
===
1. Login as "Portal"
2. Go to "/my/opportunity"
3. Create an opportunity
4. Edit this opportunity from the frontend and set the probability empty
=> Save, an error is raised
Technical
=========
The probability is set to "None" and not to "False", therefor the
verification "probability >= 100" in the write method of the lead will
raise an error.
By using False instead of None, this verification will work.
Task-2613208
closesodoo/odoo#75100
X-original-commit: e58048350b670b88c31c88cc57cdf6cc38b22f80
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>