ESQL: Refine assertion in Driver (#130127)

Refines an assertion that the next operator is never finished, which is actually possible if the next operator is an ExchangeSinkOperator.

Author: GalLalouche

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/operator/Driver.java

@@ -267,7 +267,8 @@ private IsBlockedResult runSingleLoopIteration() {
 
             if (op.isFinished() == false && nextOp.needsInput()) {
                 driverContext.checkForEarlyTermination();
-                assert nextOp.isFinished() == false : "next operator should not be finished yet: " + nextOp;
+                assert nextOp.isFinished() == false || nextOp instanceof ExchangeSinkOperator
+                    : "next operator should not be finished yet: " + nextOp;
                 Page page = op.getOutput();
                 if (page == null) {
                     // No result, just move to the next iteration