Add test case description

Signed-off-by: SirWayne <dennis.melzer@bosch-si.com>

hawkbit-dmf-amqp/src/main/java/org/eclipse/hawkbit/amqp/BaseAmqpService.java

@@ -99,6 +99,7 @@ public class BaseAmqpService {
         final Object value = header.get(key);
         if (value == null) {
             logAndThrowMessageError(message, errorMessageIfNull);
+            return null;
         }
         return value.toString();
     }