Merge pull request 'master' (#261) from master into Docker

Reviewed-on: SGD-Nord/konova#261

intervention/utils/egon_export.py

@@ -67,6 +67,7 @@ class EgonGmlBuilder:
     """
     intervention = None
     gml = None
+    _PAYMENT_FALLBACK_DATE = ""
 
     def __init__(self, intervention):
         self.intervention = intervention
@@ -164,7 +165,10 @@ class EgonGmlBuilder:
         payment_date = None
         if payment is not None:
             payment_date = payment.due_on
+            if payment_date is not None:
                 payment_date = payment_date.strftime(DEFAULT_DATE_FORMAT)
+            else:
+                payment_date = self._PAYMENT_FALLBACK_DATE
 
         cons_office = self.intervention.responsible.conservation_office
         reg_office = self.intervention.responsible.registration_office