[FIX] ir.config.parameter: set_param with None/False as value

Use `ir.config.parameter`.`set_param` with `None` or `False`
was not allowed as `value` of a `ir.config.parameter` is required.

An attempt to set a system parameter to `None` or `False` means,
most of the time, that the system parameter should be dropped.

For instance, in configuration wizards (`res.config`),
`set_param` is used when boxes are checked, or fields are filled,
to create/update system parameters with the value given in the
configuration wizards.

When unchecking these boxes, the configurations wizards tried
to set `None` in the system parameters, leading to a warning
saying the value is mandatory. The behavior expected when
unchecking these boxes was to remove the system parameters,
not to leave them empty. Besides, there is not methods
available in `ir.config.parameter` to quickly remove a system
parameter, like there is one to set a parameter.

opw-653547

openerp/addons/base/ir/ir_config_parameter.py

@@ -96,11 +96,15 @@ class ir_config_parameter(osv.osv):
         if ids:
             param = self.browse(cr, uid, ids[0], context=context)
             old = param.value
-            self.write(cr, uid, ids, vals, context=context)
+            if value is not False and value is not None:
+                self.write(cr, uid, ids, vals, context=context)
+            else:
+                self.unlink(cr, uid, ids, context=context)
             return old
         else:
             vals.update(key=key)
-            self.create(cr, uid, vals, context=context)
+            if value is not False and value is not None:
+                self.create(cr, uid, vals, context=context)
             return False
 
     def write(self, cr, uid, ids, vals, context=None):