Check for last chunk before acquiring lock

[SirYwell]

Overview

Description

Acquiring/releasing the lock in getOrCreateChunk takes a fair amount of time, even with no contention involved. By simplifying from two volatile variables to one, we can already have a fast path before the lock, This cuts the time spent in the method in half in my measurements.

One downside is the scenario where many calls result in NullChunk.getInstance() being returned, as we can't check for the coordinates there. If that is a problem, it might make sense to just create multiple NullChunk objects with their actual coordinates.

### Submitter Checklist
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- [x] Ensure that the pull request title represents the desired changelog entry.
- [x] New public fields and methods are annotated with `@since TODO`.
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[dordsor21]

I don't think there's any reason for NullChunk to be an issue