[core] Convert null to empty string, not “null”

platform/android/CHANGELOG.md

@@ -2,6 +2,9 @@
 
 Mapbox welcomes participation and contributions from everyone.  If you'd like to do so please see the [`Contributing Guide`](https://github.com/mapbox/mapbox-gl-native/blob/master/CONTRIBUTING.md) first to get started.
 
+## 6.1.2
+ - `"to-string"` expression operator converts `null` to empty string rather than to `"null"` [#11904](https://github.com/mapbox/mapbox-gl-native/pull/11904)
+
 ## 6.1.1 - May 7, 2018
  - Update telemetry to 3.1.0 [#11855](https://github.com/mapbox/mapbox-gl-native/pull/11855)
 

platform/ios/CHANGELOG.md

@@ -12,7 +12,7 @@ Mapbox welcomes participation and contributions from everyone. Please read [CONT
 
 * Deprecated `+[NSExpression featurePropertiesVariableExpression]` use `+[NSExpression featureAttributesVariableExpression]` instead. ([#11748](https://github.com/mapbox/mapbox-gl-native/pull/11748))
 * Added `FIRST`, `LAST`, and `SIZE` symbolic array subscripting support to expressions. ([#11770](https://github.com/mapbox/mapbox-gl-native/pull/11770))
-
+* Inside an expression, casting `nil` to a string turns it into the empty string instead of the string `"null"`. ([#11904](https://github.com/mapbox/mapbox-gl-native/pull/11904))
 
 ### Annotations
 

platform/macos/CHANGELOG.md

@@ -7,6 +7,7 @@
 * Deprecated `+[NSExpression featurePropertiesVariableExpression]` use `+[NSExpression featureAttributesVariableExpression]` instead. ([#11748](https://github.com/mapbox/mapbox-gl-native/pull/11748))
 * Added `-[NSPredicate(MGLAdditions) predicateWithMGLJSONObject:]` method and `NSPredicate.mgl_jsonExpressionObject` property. ([#11810](https://github.com/mapbox/mapbox-gl-native/pull/11810))
 * Added `FISRT`, `LAST`, and `SIZE` symbolic array subscripting support to expressions. ([#11770](https://github.com/mapbox/mapbox-gl-native/pull/11770))
+* Inside an expression, casting `nil` to a string turns it into the empty string instead of the string `"null"`. ([#11904](https://github.com/mapbox/mapbox-gl-native/pull/11904))
 
 ### Other changes
 

src/mbgl/style/expression/compound_expression.cpp

@@ -303,6 +303,7 @@ std::unordered_map<std::string, CompoundExpressionRegistry::Definition> initiali
 
     define("to-string", [](const Value& value) -> Result<std::string> {
         return value.match(
+            [](const NullValue&) -> Result<std::string> { return std::string(); },
             [](const Color& c) -> Result<std::string> { return c.stringify(); }, // avoid quoting
             [](const std::string& s) -> Result<std::string> { return s; }, // avoid quoting
             [](const auto& v) -> Result<std::string> { return stringify(v); }