added ParameterizedTest for offer validator

src/main/java/de/rwu/easydrop/service/validation/OfferValidator.java

@@ -28,6 +28,9 @@ public final class OfferValidator {
             if (offer.getOfferId().equals("")) {
                 throw new InvalidOfferException("Offer ID cannot be empty");
             }
+            if (offer.getLastUpdate().equals("")) {
+                throw new InvalidOfferException("LastUpdate cannot be empty");
+            }
         } catch (NullPointerException e) {
             throw new InvalidOfferException("Required information is missing in the offer", e);
         }