Don't save single value catFeaturePerfectHash to ensure that constant categ features are ignored when loading. MLTOOLS-5191.

ref:413195f165fdea146eb6e9b06876441493c1de6d

Author: andrey-khropov

catboost/private/libs/quantized_pool/serialization.cpp

@@ -1022,22 +1022,24 @@ namespace NCB {
                 const auto catFeatureIdx =
                     featuresLayout->GetInternalFeatureIdx<EFeatureType::Categorical>(externalFeatureIdx);
 
-                const auto& srcCatFeaturePerfectHash =
-                    quantizedFeaturesInfo->GetCategoricalFeaturesPerfectHash(catFeatureIdx);
-
-                TMap<ui32, TValueWithCount> dstCatFeaturePerfectHash = srcCatFeaturePerfectHash.Map;
-                if (srcCatFeaturePerfectHash.DefaultMap.Defined()) {
-                    dstCatFeaturePerfectHash.emplace(
-                        srcCatFeaturePerfectHash.DefaultMap->SrcValue,
-                        srcCatFeaturePerfectHash.DefaultMap->DstValueWithCount
-                   );
-                }
+                TMap<ui32, TValueWithCount> dstCatFeaturePerfectHash;
 
                 TMaybeData<const IQuantizedCatValuesHolder*> feature =
                     quantizedObjectsData->GetCatFeature(*catFeatureIdx);
 
                 THolder<TSrcColumnBase> maybeFeatureColumn;
                 if (feature) {
+                    const auto& srcCatFeaturePerfectHash =
+                        quantizedFeaturesInfo->GetCategoricalFeaturesPerfectHash(catFeatureIdx);
+
+                    dstCatFeaturePerfectHash = srcCatFeaturePerfectHash.Map;
+                    if (srcCatFeaturePerfectHash.DefaultMap.Defined()) {
+                        dstCatFeaturePerfectHash.emplace(
+                            srcCatFeaturePerfectHash.DefaultMap->SrcValue,
+                            srcCatFeaturePerfectHash.DefaultMap->DstValueWithCount
+                       );
+                    }
+
                     if (dstCatFeaturePerfectHash.size() > ((size_t)Max<ui16>() + 1)) {
                         maybeFeatureColumn = GenerateSrcColumn<ui32>(**feature);
                     } else if (dstCatFeaturePerfectHash.size() > ((size_t)Max<ui8>() + 1)) {