Fix NPE in treeFinished()

tryPushTasks returns null when there are no tasks, not
an empty list.

patches/server/0005-Increase-parallelism-for-neighbour-writing-chunk-sta.patch

@@ -341,7 +341,7 @@ index 73ce0909bd89244835a0d0f2030a25871461f1e0..ecc366a4176b2efadc46aa91aa21621f
      @Override
 diff --git a/src/main/java/io/papermc/paper/chunk/system/scheduling/queue/RadiusAwarePrioritisedExecutor.java b/src/main/java/io/papermc/paper/chunk/system/scheduling/queue/RadiusAwarePrioritisedExecutor.java
 new file mode 100644
-index 0000000000000000000000000000000000000000..f3ec7a7fcfee31b618104499449643baea602478
+index 0000000000000000000000000000000000000000..6648f8cc5c1788ec02a3adbb68cf126372017dd3
 --- /dev/null
 +++ b/src/main/java/io/papermc/paper/chunk/system/scheduling/queue/RadiusAwarePrioritisedExecutor.java
 @@ -0,0 +1,664 @@
@@ -384,7 +384,7 @@ index 0000000000000000000000000000000000000000..f3ec7a7fcfee31b618104499449643ba
 +            if (queue.hasWaitingTasks()) {
 +                final List<PrioritisedExecutor.PrioritisedTask> ret = queue.tryPushTasks();
 +
-+                if (ret.isEmpty()) {
++                if (ret == null || ret.isEmpty()) {
 +                    // this happens when the tasks in the wait queue were purged
 +                    // in this case, the queue was actually empty, we just had to purge it
 +                    // if we set the selected queue without scheduling any tasks, the queue will never be unselected