fix JsonValue.asString() invocation when value was no string (e.g. `n…

…ull`)

Signed-off-by: Thomas Jaeckle <thomas.jaeckle@bosch.io>

policies/model/src/main/java/org/eclipse/ditto/policies/model/ImmutableSubjectAnnouncement.java

@@ -156,6 +156,7 @@ public String toString() {
 
     private static List<AcknowledgementRequest> deserializeRequestedAcks(final JsonArray jsonArray) {
         return jsonArray.stream()
+                .filter(JsonValue::isString)
                 .map(value -> AcknowledgementRequest.parseAcknowledgementRequest(value.asString()))
                 .collect(Collectors.toList());
     }