JSONFilter should support null value

[fhussonnois]

io.streamthoughts.kafka.connect.filepulse.filter.FilterException: Error while reading json value, invalid JSON message.
at io.streamthoughts.kafka.connect.filepulse.filter.JSONFilter.apply(JSONFilter.java:71)
at io.streamthoughts.kafka.connect.filepulse.filter.AbstractMergeRecordFilter.apply(AbstractMergeRecordFilter.java:51)
at io.streamthoughts.kafka.connect.filepulse.filter.DefaultRecordFilterPipeline$FilterNode.apply(DefaultRecordFilterPipeline.java:159)
at io.streamthoughts.kafka.connect.filepulse.filter.DefaultRecordFilterPipeline.apply(DefaultRecordFilterPipeline.java:131)
at io.streamthoughts.kafka.connect.filepulse.filter.DefaultRecordFilterPipeline.apply(DefaultRecordFilterPipeline.java:99)
at io.streamthoughts.kafka.connect.filepulse.source.DefaultFileRecordsPollingConsumer.next(DefaultFileRecordsPollingConsumer.java:169)
at io.streamthoughts.kafka.connect.filepulse.source.FilePulseSourceTask.poll(FilePulseSourceTask.java:126)
at org.apache.kafka.connect.runtime.WorkerSourceTask.poll(WorkerSourceTask.java:272)
at org.apache.kafka.connect.runtime.WorkerSourceTask.execute(WorkerSourceTask.java:239)
at org.apache.kafka.connect.runtime.WorkerTask.doRun(WorkerTask.java:184)
at org.apache.kafka.connect.runtime.WorkerTask.run(WorkerTask.java:234)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
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)
Caused by: io.streamthoughts.kafka.connect.filepulse.reader.ReaderException: Error while reading value in JSON - Unknown type NULL
at io.streamthoughts.kafka.connect.filepulse.json.DefaultJSONStructConverter.getAccessorForType(DefaultJSONStructConverter.java:58)
at io.streamthoughts.kafka.connect.filepulse.json.DefaultJSONStructConverter.access$500(DefaultJSONStructConverter.java:35)
at io.streamthoughts.kafka.connect.filepulse.json.DefaultJSONStructConverter$ObjectJsonFieldAccessor.read(DefaultJSONStructConverter.java:151)
at io.streamthoughts.kafka.connect.filepulse.json.DefaultJSONStructConverter.readJson(DefaultJSONStructConverter.java:73)
at io.streamthoughts.kafka.connect.filepulse.filter.JSONFilter.apply(JSONFilter.java:69)