[FIX] project_issue: correct onchange_project_id (2)

amend to d78256e7

We wish to keep the email address if no partner is assigned.

addons/project_issue/models/project_issue.py

@@ -151,7 +151,7 @@ class ProjectIssue(models.Model):
     @api.onchange('project_id')
     def _onchange_project_id(self):
         if self.project_id:
-            if not self.partner_id:
+            if not self.partner_id and not self.email_from:
                 self.partner_id = self.project_id.partner_id.id
                 self.email_from = self.project_id.partner_id.email
             self.stage_id = self.stage_find(self.project_id.id, [('fold', '=', False)])