extend ignoring of transactionIds to all negative values (#1415)

#1415

Author: goekay

src/main/java/de/rwth/idsg/steve/service/CentralSystemService16_Service.java

@@ -279,7 +279,10 @@ public DataTransferResponse dataTransfer(DataTransferRequest parameters, String
      */
     private Integer getTransactionId(MeterValuesRequest parameters) {
         Integer transactionId = parameters.getTransactionId();
-        if (transactionId == null || transactionId == 0) {
+        if (transactionId == null) {
+            return null;
+        } else if (transactionId < 1) {
+            log.warn("MeterValues transactionId is invalid ({}), ignoring it", transactionId);
             return null;
         }
         return transactionId;