SuperSorter: Don't set allDone if it's already set. (apache#17238)

This fixes a race where, if there is no output at all, setAllDoneIfPossible
could be called twice (once when the output partitions future resolves, and
once when the batcher finishes). If the calls happen in that order, it would
try to create nil output channels both times, resulting in a "Channel already set"
error.

processing/src/main/java/org/apache/druid/frame/processor/SuperSorter.java

@@ -391,6 +391,11 @@ private void runWorkersIfPossible()
   @GuardedBy("runWorkersLock")
   private void setAllDoneIfPossible()
   {
+    if (isAllDone()) {
+      // Already done, no need to set allDone again.
+      return;
+    }
+
     try {
       if (totalInputFrames == 0 && outputPartitionsFuture.isDone()) {
         // No input data -- generate empty output channels.