Fix NullPointerException in Monitor.getQuery when query is not present

[daviscook477]

Description

The javadoc for Monitor.getQuery suggests that it will return null when the query is not present but instead it throws a NullPointerException.

This is because bytesHolder[0] will still be null after the call to search if it does not find any query matching the id in the monitor's index. Then when the serializer.deserialize call is made, any attempts to index the BytesRef will throw the NullPointerException. This will happen here when using the serializer/deserializer created with MonitorQuerySerializer.fromParser.

Alternative Approach

This could instead be fixed just for the case of MonitorQuerySerializer.fromParser by making the serializer/deserializer returned from it null-safe (i.e. it checks if the input is null and returns null). But I went with this approach since it would be robust against custom implementations of the MonitorQuerySerializer interface that also do not check for null like the fromParser implementation.

Tests

I introduced testGetQueryNotPresent which fails on master with the NullPointerException, but passes on this branch. I also added a corresponding testGetQueryPresent which passes on both master and this branch. Since I needed a monitor with persistence for these two new tests I refactored creating a monitor with persistence into a helper function newMonitorWithPersistence that gets reused throughout the TestMonitorPersistence file.

[romseygeek]

This looks great, thank you @daviscook477! Would you be able to add an entry to CHANGES.txt under the 9.9.0 release?

[daviscook477]

@romseygeek I have added a CHANGES.txt entry under the 9.9.0 release describing the bugfix in this PR.

[romseygeek]

Thanks @daviscook477!