SAMZA-2802: fetch empty topic metadata

[Anilochka]

In the situation when a topic is empty, but not new (begin offset == end offset != 0) in fetchTopicPartitionsMetadata we subtract 1 from the end offset:
newestOffsets.put(KafkaUtil.toSystemStreamPartition(this.systemName, topicPartition), String.valueOf(offset - 1L));
and later in validateRestoreOffsets:

private static void validateRestoreOffsets(RestoreOffsets restoreOffsets, SystemAdmin systemAdmin) {
    String startingOffset = restoreOffsets.startingOffset;
    String endingOffset = restoreOffsets.endingOffset;
    // cannot enforce that starting offset be non null since SSPMetadata allows oldest offset == null for empty topics.
    if (endingOffset != null) {
      Preconditions.checkState(systemAdmin.offsetComparator(endingOffset, startingOffset) >= 0,
          String.format("Ending offset: %s must be equal to or greater than starting offset: %s",
              endingOffset, startingOffset));
    }
  }

we get endingOffset < startingOffset because of subtraction of 1 in fetchTopicPartitionsMetadata.

In this PR if-else block was added to don't subtract 1 from the ending offset when its equal to the beginning offset.