yql_kikimr_datasource RewriteIO: avoid sigsegv on missing Metadata (backport 25-1 #26795) (#27404)

Author: yumkam

ydb/core/kqp/provider/yql_kikimr_datasource.cpp

@@ -710,6 +710,7 @@ class TKikimrDataSource : public TDataProviderBase {
         const TString tablePath = key.GetTablePath();
         auto& tableDesc = SessionCtx->Tables().GetTable(cluster, tablePath);
         if (key.GetKeyType() == TKikimrKey::Type::Table) {
+            YQL_ENSURE(tableDesc.Metadata);
             if (tableDesc.Metadata->Kind == EKikimrTableKind::External) {
                 if (tableDesc.Metadata->ExternalSource.SourceType == ESourceType::ExternalDataSource && tableDesc.Metadata->TableType == NYql::ETableType::Unknown) {
                     ctx.AddError(TIssue(node->Pos(ctx),