[core] Fix TransitionOptions serialization

include/mbgl/style/conversion_impl.hpp

@@ -306,14 +306,7 @@ struct ValueFactory<ColorRampPropertyValue> {
 
 template <>
 struct ValueFactory<TransitionOptions> {
-    static Value make(const TransitionOptions& value) {
-        return mapbox::base::ValueArray{
-            {std::chrono::duration_cast<std::chrono::milliseconds>(value.duration.value_or(mbgl::Duration::zero()))
-                 .count(),
-             std::chrono::duration_cast<std::chrono::milliseconds>(value.delay.value_or(mbgl::Duration::zero()))
-                 .count(),
-             value.enablePlacementTransitions}};
-    }
+    static Value make(const TransitionOptions& value) { return value.serialize(); }
 };
 
 template <>
@@ -370,10 +363,12 @@ StyleProperty makeStyleProperty(const PropertyValue<T>& value) {
 }
 
 inline StyleProperty makeStyleProperty(const TransitionOptions& value) {
+    if (!value.isDefined()) return {};
     return {makeValue(value), StyleProperty::Kind::Transition};
 }
 
 inline StyleProperty makeStyleProperty(const ColorRampPropertyValue& value) {
+    if (value.isUndefined()) return {};
     return {makeValue(value), StyleProperty::Kind::Expression};
 }
 

include/mbgl/style/transition_options.hpp

@@ -3,6 +3,8 @@
 #include <mbgl/util/chrono.hpp>
 #include <mbgl/util/optional.hpp>
 
+#include <mapbox/value.hpp>
+
 namespace mbgl {
 namespace style {
 
@@ -30,6 +32,17 @@ public:
     bool isDefined() const {
         return duration || delay;
     }
+
+    mapbox::base::Value serialize() const {
+        mapbox::base::ValueObject result;
+        if (duration) {
+            result.emplace("duration", std::chrono::duration_cast<std::chrono::milliseconds>(*duration).count());
+        }
+        if (delay) {
+            result.emplace("delay", std::chrono::duration_cast<std::chrono::milliseconds>(*delay).count());
+        }
+        return result;
+    }
 };
 
 } // namespace style