Fixed segfault for STATS_COLLECTION_FULL

Author: GrigoriyPA

ydb/core/kqp/session_actor/kqp_session_actor.cpp

@@ -1605,7 +1605,9 @@ class TKqpSessionActor : public TActorBootstrapped<TKqpSessionActor> {
             auto stats = QueryState->QueryStats.ToProto();
             if (QueryState->GetStatsMode() >= Ydb::Table::QueryStatsCollection::STATS_COLLECTION_FULL) {
                 response->SetQueryPlan(SerializeAnalyzePlan(stats, QueryState->UserRequestContext->PoolId));
-                response->SetQueryAst(QueryState->CompileResult->PreparedQuery->GetPhysicalQuery().GetQueryAst());
+                if (QueryState->CompileResult && QueryState->CompileResult->PreparedQuery) {
+                    response->SetQueryAst(QueryState->CompileResult->PreparedQuery->GetPhysicalQuery().GetQueryAst());
+                }
             }
             response->MutableQueryStats()->Swap(&stats);
         }