[FIX] project_issue: don't reset partner on project change

If an issue project is changed, depending if the destination project has
a set partner ("Customer") or not:

- if set, the project issue partner will be set accordingly
- otherway the potentially set partner will be conserved

Since 3fe06da if the project had not partner, the partner was unset
which is undesired.

addons/project_issue/project_issue.py

@@ -124,7 +124,7 @@ class project_issue(osv.Model):
             project = self.pool.get('project.project').browse(cr, uid, project_id, context=context)
             if project and project.partner_id:
                 return {'value': {'partner_id': project.partner_id.id}}
-        return {'value': {'partner_id': False}}
+        return {}
 
     _columns = {
         'id': fields.integer('ID', readonly=True),