[FIX] hr_payroll: Fix localdict ambiguity on rule computation

Purpose
=======

On a payslip rule computation you can return 3 parameters:
- result_qty
- result_rate
- result

From this, we compute the total amount as following:

tot_rule = amount * qty * rate / 100.0

If we set parameters result_qty=0 or result_rate=0 to force the tot_rule to
be equal to 0, then their values are set to the default one, which is ambiguous.

So
result_qty = 0
result_rate = 0

Become
qty=1
amount=100

We can't fix this in a stable release as this could imply unwanted
behavior changes for customers, and also quite difficult to spot.

closes odoo/odoo#29206

addons/hr_payroll/models/hr_salary_rule.py

@@ -211,7 +211,7 @@ class HrSalaryRule(models.Model):
         else:
             try:
                 safe_eval(self.amount_python_compute, localdict, mode='exec', nocopy=True)
-                return float(localdict['result']), 'result_qty' in localdict and localdict['result_qty'] or 1.0, 'result_rate' in localdict and localdict['result_rate'] or 100.0
+                return float(localdict['result']), localdict.get('result_qty', 1.0), localdict.get('result_rate', 100.0)
             except Exception as e:
                 raise UserError(_('Wrong python code defined for salary rule %s (%s).\nError: %s') % (self.name, self.code, e))
 
@@ -234,7 +234,7 @@ class HrSalaryRule(models.Model):
         else:  # python code
             try:
                 safe_eval(self.condition_python, localdict, mode='exec', nocopy=True)
-                return 'result' in localdict and localdict['result'] or False
+                return  localdict.get('result', False)
             except:
                 raise UserError(_('Wrong python condition defined for salary rule %s (%s).') % (self.name, self.code))