Fix NetworkTopologyImpl#getLeaves returning set with null value in case of non existing scope

Descriptions of the changes in this PR:

NetworkTopologyImpl#getLeaves(String scope) was returning set with null value, when requested scope did not exist.
I've added null check in NetworkTopologyImpl#doGetLeaves(String scope), used by NetworkTopologyImpl#getLeaves method to not have null node returned as leaf node.

It would be nice to have this merged also with non master branch of bookeeper so maintenance releases could have that fix.

### Motivation
I've found my Apache Pulsar cluster polluted with log:
```
 12:01:46.462 pulsar2-dev pulsar-broker {"logLevel":"ERROR","logThread":"pulsar-io-23-1","logger":"org.apache.bookkeeper.client.RackawareEnsemblePlacementPolicyImpl","message":"found non-BookieNode: null as leaf of defaultrack: /default-rack","stack_trace":null}
```

After digging I've found that, because https://github.com/apache/bookkeeper/blob/master/bookkeeper-server/src/main/java/org/apache/bookkeeper/client/RackawareEnsemblePlacementPolicyImpl.java#L324
was returning HashSet with null node, when no bookkeepers were in /default-rack.

### Changes
- Modified doGetLeaves() method to return empty HashSet instead of HashSet with null element when scope node can't be found.



Reviewers: Enrico Olivelli <eolivelli@gmail.com>, Andrey Yegorov <andrey.yegorov@datastax.com>

This closes #2632 from mkozioro/fix-do-get-leaves
2 files changed
tree: d1b8405bd671ab62b6ca2f5fd6f277c2d54c3511
  1. .github/
  2. .gitignore
  3. .test-infra/
  4. Jenkinsfile
  5. LICENSE
  6. NOTICE
  7. README.md
  8. bin/
  9. bookkeeper-benchmark/
  10. bookkeeper-common-allocator/
  11. bookkeeper-common/
  12. bookkeeper-dist/
  13. bookkeeper-http/
  14. bookkeeper-proto/
  15. bookkeeper-server/
  16. bookkeeper-stats-providers/
  17. bookkeeper-stats/
  18. buildtools/
  19. circe-checksum/
  20. conf/
  21. cpu-affinity/
  22. deploy/
  23. dev/
  24. docker/
  25. metadata-drivers/
  26. microbenchmarks/
  27. pom.xml
  28. shaded/
  29. site/
  30. site2/
  31. stats/
  32. stream/
  33. tests/
  34. tools/
README.md

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