Handle oprional in ConversionTraits::GetEntry

Author: Rafał Hibner

cpp/src/arrow/stl.h

@@ -222,6 +222,7 @@ struct ConversionTraits<Optional, enable_if_optional_like<Optional>>
   using OptionalInnerType =
       typename std::decay<decltype(*std::declval<Optional>())>::type;
   using typename CTypeTraits<OptionalInnerType>::ArrowType;
+  using typename CTypeTraits<OptionalInnerType>::ArrayType;
   using CTypeTraits<OptionalInnerType>::type_singleton;
 
   static Status AppendRow(typename TypeTraits<ArrowType>::BuilderType& builder,
@@ -232,6 +233,14 @@ struct ConversionTraits<Optional, enable_if_optional_like<Optional>>
       return builder.AppendNull();
     }
   }
+
+  static Optional GetEntry(const ArrayType& array, size_t j) {
+    if (array.IsNull(j)) {
+      return {};
+    } else {
+      return ConversionTraits<OptionalInnerType>::GetEntry(array, j);
+    }
+  }
 };
 
 /// Build an arrow::Schema based upon the types defined in a std::tuple-like structure.