@Override
    void callObserver() throws IOException {
      Optional<O> observer = observerGetter.apply(getEnvironment().getInstance());
      if (observer.isPresent()) {
        call(observer.get());
      }
    }
In case of coprocessors which have many kinds of observers (for eg, {@link RegionCoprocessor}
has BulkLoadObserver, RegionObserver, etc), some implementations may not need all observers,
in which case they will return null for that observer's getter. We simply ignore such cases.
hbase-server/src/main/java/org/apache/hadoop/hbase/coprocessor/CoprocessorHost.java