[controller] Return null instead of Collections.emptySet() (#634)

In isReplicaDisabled returning emptySet will cause java.lang.UnsupportedOperationException: null exception to be thrown if we try to add any element to it
Instead return null so that it will later create a new HashSet when needed.

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Co-authored-by: Sourav Maji <[email protected]>

services/venice-controller/src/main/java/com/linkedin/venice/pushmonitor/AbstractPushMonitor.java

@@ -762,14 +762,12 @@ public boolean isReplicaDisabled(String instance, int partitionId) {
         Set<Integer> disabledPartitions = disabledReplicaMap.computeIfAbsent(instance, k -> {
           helixClientThrottler.maybeThrottle(1);
           Map<String, List<String>> helixMap = helixAdminClient.getDisabledPartitionsMap(clusterName, instance);
-          if (helixMap.containsKey(kafkaTopic)) {
-            return helixMap.get(kafkaTopic).stream().map(HelixUtils::getPartitionId).collect(Collectors.toSet());
-          } else {
-            return Collections.emptySet();
-          }
+          List<String> disablePartitionList = helixMap.get(kafkaTopic);
+          return disablePartitionList != null
+              ? disablePartitionList.stream().map(HelixUtils::getPartitionId).collect(Collectors.toSet())
+              : null;
         });
-
-        return disabledPartitions.contains(partitionId);
+        return disabledPartitions != null && disabledPartitions.contains(partitionId);
       }
     };
   }