Minor fix for visibility Rules Service

dmp-backend/web/src/main/java/eu/eudat/logic/services/forms/VisibilityRuleServiceImpl.java

@@ -51,7 +51,10 @@ public class VisibilityRuleServiceImpl implements VisibilityRuleService {
         if (values instanceof List) {
             return ((Collection<?>) values).contains(source);
         } else {
+            if (values != null) {
                 return values.equals(source);
             }
+            return false;
+        }
     }
 }