FireHydrant: Return empty from getSegmentForQuery when adapter is null.

This can happen if the segment is removed while a query is in progress.
Returning empty causes the server to use ReportTimelineMissingSegmentQueryRunner,
which causes the Broker to look for the segment somewhere else.

Fixes apache#12168.

server/src/main/java/org/apache/druid/segment/realtime/FireHydrant.java

@@ -175,6 +175,12 @@ public Optional<Pair<SegmentReference, Closeable>> getSegmentForQuery(
   )
   {
     ReferenceCountingSegment sinkSegment = adapter.get();
+
+    if (sinkSegment == null) {
+      // adapter can be null if this segment is removed while being queried.
+      return Optional.empty();
+    }
+
     SegmentReference segment = segmentMapFn.apply(sinkSegment);
     while (true) {
       Optional<Closeable> reference = segment.acquireReferences();

server/src/test/java/org/apache/druid/segment/realtime/FireHydrantTest.java

@@ -146,6 +146,22 @@ public void testGetSegmentForQuerySwapped() throws IOException
     Assert.assertEquals(0, queryableSegmentReference.getNumReferences());
   }
 
+  @Test
+  public void testGetSegmentForQuerySwappedWithNull() throws IOException
+  {
+    ReferenceCountingSegment incrementalSegmentReference = hydrant.getHydrantSegment();
+    hydrant.swapSegment(null);
+    ReferenceCountingSegment queryableSegmentReference = hydrant.getHydrantSegment();
+    Assert.assertEquals(0, incrementalSegmentReference.getNumReferences());
+    Assert.assertNull(queryableSegmentReference);
+
+    Optional<Pair<SegmentReference, Closeable>> maybeSegmentAndCloseable = hydrant.getSegmentForQuery(
+        Function.identity()
+    );
+    Assert.assertEquals(0, incrementalSegmentReference.getNumReferences());
+    Assert.assertFalse(maybeSegmentAndCloseable.isPresent());
+  }
+
   @Test
   public void testGetSegmentForQueryButNotAbleToAcquireReferences()
   {