[FLINK-22545][tests] Fix delayed split assignment in OperatorEventSendingCheckpointITCase

Author: StephanEwen

flink-tests/src/test/java/org/apache/flink/runtime/operators/coordination/OperatorEventSendingCheckpointITCase.java

@@ -280,6 +280,10 @@ public Collection<SplitT> snapshotState() throws Exception {
 
         private void fullFillPendingRequests() {
             for (int subtask : pendingRequests) {
+                // respond only to requests for which we still have registered readers
+                if (!context.registeredReaders().containsKey(subtask)) {
+                    continue;
+                }
                 super.handleSplitRequest(subtask, null);
             }
             pendingRequests.clear();