GREG-21: Allow start date for roles to be null

greg/models.py

@@ -93,7 +93,9 @@ class PersonRole(BaseModel):
     unit = models.ForeignKey(
         "OrganizationalUnit", on_delete=models.PROTECT, related_name="unit_person_role"
     )
-    start_date = models.DateField()
+    # The start date can be null for people that are already
+    # attached to the institution but does not have a guest account
+    start_date = models.DateField(null=True)
     end_date = models.DateField()
     # TODO Is this field needed?
     contact_person_unit = models.TextField()