fix sl on empty inputs (ydb-platform#27588)

Author: gridnevvvit

ydb/core/kqp/runtime/kqp_stream_lookup_actor.cpp

@@ -610,6 +610,13 @@ class TKqpStreamLookupActor : public NActors::TActorBootstrapped<TKqpStreamLooku
         auto guard = BindAllocator();
 
         NUdf::TUnboxedValue row;
+
+        YQL_ENSURE(!Input.IsInvalid());
+        if (Input.IsFinish() || !Input.HasValue()) {
+            LastFetchStatus = NUdf::EFetchStatus::Finish;
+            return;
+        }
+
         while ((LastFetchStatus = Input.Fetch(row)) == NUdf::EFetchStatus::Ok) {
             StreamLookupWorker->AddInputRow(std::move(row));
         }

ydb/core/kqp/ut/join/kqp_index_lookup_join_ut.cpp

@@ -240,12 +240,10 @@ class TTester {
     bool OnlineReadOnly = false;
     NYdb::TParamsBuilder ParamsBuilder;
     bool NeedParams = false;
-    bool FilterOptionalSide = true;
 
     TTester& Run() {
         auto settings = TKikimrSettings();
         settings.AppConfig.MutableTableServiceConfig()->SetEnableKqpDataQueryStreamIdxLookupJoin(StreamLookup);
-        settings.AppConfig.MutableTableServiceConfig()->SetFilterPushdownOverJoinOptionalSide(FilterOptionalSide);
 
         TKikimrRunner kikimr(settings);
         auto db = kikimr.GetTableClient();
@@ -1182,7 +1180,6 @@ Y_UNIT_TEST_TWIN(TestEntityFramework, StreamLookupJoin) {
         ])",
         .StreamLookup=StreamLookupJoin,
         .DoValidateStats=false,
-        .FilterOptionalSide=false // remove this to get failure
     };
     tester.Run();
 }