Fix null pointer exception in HllCountInitFn

When trying to aggregate input with null value we will fail on null pointer exception, when we deal with high events rate we can have sometimes "broken" events and I don't want them the break all the pipline.

trace:
exception: "java.lang.NullPointerException
at com.google.zetasketch.HyperLogLogPlusPlus.add(HyperLogLogPlusPlus.java:212)
at org.apache.beam.sdk.extensions.zetasketch.HllCountInitFn$ForString.addInput(HllCountInitFn.java:147)
at org.apache.beam.sdk.extensions.zetasketch.HllCountInitFn$ForString.addInput(HllCountInitFn.java:137)
at org.apache.beam.sdk.extensions.sql.impl.transform.agg.AggregationCombineFnAdapter$WrappedCombinerBase.addInput(AggregationCombineFnAdapter.java:54)
at org.apache.beam.sdk.transforms.CombineFns$ComposedCombineFn.addInput(CombineFns.java:382)
at org.apache.beam.sdk.schemas.transforms.SchemaAggregateFn$Inner.addInput(SchemaAggregateFn.java:324)
at org.apache.beam.sdk.schemas.transforms.SchemaAggregateFn$Inner.addInput(SchemaAggregateFn.java:63)
at org.apache.beam.runners.dataflow.worker.WindmillStateInternals$WindmillCombiningState.add(WindmillStateInternals.java:2056)
at org.apache.beam.runners.dataflow.worker.repackaged.org.apache.beam.runners.core.SystemReduceFn.processValue(SystemReduceFn.java:119)
at org.apache.beam.runners.dataflow.worker.repackaged.org.apache.beam.runners.core.ReduceFnRunner.processElement(ReduceFnRunner.java:613)
at org.apache.beam.runners.dataflow.worker.repackaged.org.apache.beam.runners.core.ReduceFnRunner.processElements(ReduceFnRunner.java:360)
at org.apache.beam.runners.dataflow.worker.StreamingGroupAlsoByWindowViaWindowSetFn.processElement(StreamingGroupAlsoByWindowViaWindowSetFn.java:96)
at org.apache.beam.runners.dataflow.worker.StreamingGroupAlsoByWindowViaWindowSetFn.processElement(StreamingGroupAlsoByWindowViaWindowSetFn.java:43)
at org.apache.beam.runners.dataflow.worker.GroupAlsoByWindowFnRunner.invokeProcessElement(GroupAlsoByWindowFnRunner.java:121)
at org.apache.beam.runners.dataflow.worker.GroupAlsoByWindowFnRunner.processElement(GroupAlsoByWindowFnRunner.java:73)
at org.apache.beam.runners.dataflow.worker.repackaged.org.apache.beam.runners.core.LateDataDroppingDoFnRunner.processElement(LateDataDroppingDoFnRunner.java:80)
at org.apache.beam.runners.dataflow.worker.GroupAlsoByWindowsParDoFn.processElement(GroupAlsoByWindowsParDoFn.java:137)
at org.apache.beam.runners.dataflow.worker.util.common.worker.ParDoOperation.process(ParDoOperation.java:44)
at org.apache.beam.runners.dataflow.worker.util.common.worker.OutputReceiver.process(OutputReceiver.java:49)
at org.apache.beam.runners.dataflow.worker.util.common.worker.ReadOperation.runReadLoop(ReadOperation.java:212)
at org.apache.beam.runners.dataflow.worker.util.common.worker.ReadOperation.start(ReadOperation.java:163)
at org.apache.beam.runners.dataflow.worker.util.common.worker.MapTaskExecutor.execute(MapTaskExecutor.java:92)
at org.apache.beam.runners.dataflow.worker.StreamingDataflowWorker.process(StreamingDataflowWorker.java:1437)
at org.apache.beam.runners.dataflow.worker.StreamingDataflowWorker.access$1100(StreamingDataflowWorker.java:165)
at org.apache.beam.runners.dataflow.worker.StreamingDataflowWorker$7.run(StreamingDataflowWorker.java:1113)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)