Refactor addDefaultRackBookiesIfMinNumRacksIsEnforced to Remove Redundant Logic and Improve Robustness

### Motivation

The current implementation of `addDefaultRackBookiesIfMinNumRacksIsEnforced` has the following issues:

- **Redundant logic**: The initialization and merging process for collecting Bookie nodes on the default rack involves unnecessary data copying and checks.
- **Potential robustness issue**: There is no null check for the address returned by `BookieNode.getAddr()`, which may result in `null` values being included in the exclusion set.
- **Incorrect logging**: The log claims to print only the defaultRack Bookies that are being excluded, while it actually prints all the nodes, including those already in `excludeBookies`.
  ```java
  LOG.info("enforceMinNumRacksPerWriteQuorum is enabled, so Excluding bookies of defaultRack: {}", bookiesInDefaultRack);
  ```

---

### Modification

- When creating `bookiesInDefaultRack`, do not copy elements from `excludeBookies`.
- Add a null check for the address returned by `BookieNode.getAddr()`.
- Remove unnecessary null checks for `bookiesInDefaultRack`.

Author: xiangying

bookkeeper-server/src/main/java/org/apache/bookkeeper/client/RackawareEnsemblePlacementPolicyImpl.java

@@ -364,16 +364,16 @@ protected Set<BookieId> addDefaultRackBookiesIfMinNumRacksIsEnforced(
             Set<BookieId> bookiesInDefaultRack = null;
             Set<Node> defaultRackLeaves = topology.getLeaves(getDefaultRack());
             for (Node node : defaultRackLeaves) {
-                if (node instanceof BookieNode) {
+                if (node instanceof BookieNode && ((BookieNode) node).getAddr() != null) {
                     if (bookiesInDefaultRack == null) {
-                        bookiesInDefaultRack = new HashSet<BookieId>(excludeBookies);
+                        bookiesInDefaultRack = new HashSet<>();
                     }
                     bookiesInDefaultRack.add(((BookieNode) node).getAddr());
                 } else {
                     LOG.error("found non-BookieNode: {} as leaf of defaultrack: {}", node, getDefaultRack());
                 }
             }
-            if ((bookiesInDefaultRack == null) || bookiesInDefaultRack.isEmpty()) {
+            if (bookiesInDefaultRack == null) {
                 comprehensiveExclusionBookiesSet = excludeBookies;
             } else {
                 comprehensiveExclusionBookiesSet = new HashSet<BookieId>(excludeBookies);