fix streamlookup keys join on unmatch

SELECT s.id, db.id FROM yds.`in` AS s
  LEFT JOIN /*+ streamlookup() */ ydb.`db` as db
    ON s.id = db.id;
returned db.id equal to s.id on unmatched rows

Author: yumkam

ydb/library/yql/dq/actors/input_transforms/dq_input_transform_lookup.cpp

@@ -111,7 +111,7 @@ class TInputTransformStreamLookupBase
                         outputRowItems[i] = wideInputRow[index];
                         break;
                     case EOutputRowItemSource::LookupKey:
-                        outputRowItems[i] = lookupKey.GetElement(index);
+                        outputRowItems[i] = lookupPayload && *lookupPayload ? lookupKey.GetElement(index) : NUdf::TUnboxedValue {};
                         break;
                     case EOutputRowItemSource::LookupOther:
                         if (lookupPayload && *lookupPayload) {