Expose messages number

Author: germanosin

api/src/main/java/io/kafbat/ui/model/InternalTopic.java

@@ -147,6 +147,7 @@ public static InternalTopic from(TopicDescription topicDescription,
   public Long getMessagesNumber() {
     Long result = null;
     if (cleanUpPolicy.equals(CleanupPolicy.DELETE)) {
+      result = 0L;
       if (partitions != null && !partitions.isEmpty()) {
         for (InternalPartition partition : partitions.values()) {
           result += (partition.getOffsetMax() - partition.getOffsetMin());