[KQP] Multiset explain bug fix (#9866)

Author: pashandor789

ydb/core/grpc_services/query/rpc_execute_query.cpp

@@ -392,7 +392,7 @@ class TExecuteQueryRPC : public TActorBootstrapped<TExecuteQueryRPC> {
         bool hasTrailingMessage = false;
 
         auto& kqpResponse = record.GetResponse();
-        if (kqpResponse.GetYdbResults().size() > 1) {
+        if (kqpResponse.GetYdbResults().size() > 1 && QueryAction != NKikimrKqp::QUERY_ACTION_EXPLAIN) {
             auto issue = MakeIssue(NKikimrIssues::TIssuesIds::DEFAULT_ERROR,
                 "Unexpected trailing message with multiple result sets.");
             ReplyFinishStream(Ydb::StatusIds::INTERNAL_ERROR, issue);