Broker select custom priority NPE

[wjhypo]

Affected Version

Master branch

Description

1. When druid.broker.select.tier is set to custom and druid.broker.select.tier.custom.priorities is omitted, we get NPE when broker updates server view, e.g, when there's segment balance in historical nodes.
2. When druid.broker.select.tier is set to custom and druid.broker.select.tier.custom.priorities doesn't explicitly list all the priorities of existing tiers, we get NPE same as above.

Exception in thread "BrokerServerView-1985" java.lang.NullPointerException
        at java.util.Comparator.lambda$comparingInt$7b0bb60$1(Comparator.java:490)
        at it.unimi.dsi.fastutil.ints.IntComparators$1.compare(IntComparators.java:86)
        at it.unimi.dsi.fastutil.ints.Int2ObjectRBTreeMap.compare(Int2ObjectRBTreeMap.java:205)
        at it.unimi.dsi.fastutil.ints.Int2ObjectRBTreeMap.findKey(Int2ObjectRBTreeMap.java:219)
        at it.unimi.dsi.fastutil.ints.Int2ObjectRBTreeMap.get(Int2ObjectRBTreeMap.java:931)
        at it.unimi.dsi.fastutil.ints.Int2ObjectMap.computeIfAbsent(Int2ObjectMap.java:380)
        at org.apache.druid.client.selector.ServerSelector.addServerAndUpdateSegment(ServerSelector.java:72)