Do not fail to compute record name of opportunity reporting without employee

Closes #13114
(grafted from 6cdbfed2effb3f71b3f688564de4471da095546f)

--HG--
branch : 7.0

modules/sale_opportunity/opportunity_reporting.py

@@ -424,7 +424,8 @@ class EmployeeMixin:
             opportunity.employee]
 
     def get_rec_name(self, name):
-        return self.employee.rec_name
+        if self.employee:
+            return self.employee.rec_name
 
     @classmethod
     def search_rec_name(cls, name, clause):