Change ["to-string", null] from "null" to "" (#6534)

* Change ["to-string", null] from "null" to ""

Closes #6533

* Update changelog and style-spec docs

Cherry-picked from d5c5ef5.

CHANGELOG.md

@@ -3,6 +3,7 @@
 ### ⚠️ Breaking changes
 
 * `Evented` is no longer publicly exported, and `Evented#fire` and `Evented#listens` are now private. If you are writing a class that needs event emitting functionality, consider using [`EventEmitter`](https://nodejs.org/api/events.html#events_class_eventemitter) or similar libraries instead.
+* The `"to-string"` expression operator now converts `null` to an empty string rather than to `"null"`. [#6534](https://github.com/mapbox/mapbox-gl-js/pull/6534)
 
 ## 0.44.2
 

src/style-spec/expression/definitions/index.js

@@ -105,7 +105,9 @@ CompoundExpression.register(expressions, {
         (ctx, [v]) => {
             v = v.evaluate(ctx);
             const type = typeof v;
-            if (v === null || type === 'string' || type === 'number' || type === 'boolean') {
+            if (v === null) {
+                return '';
+            } else if (type === 'string' || type === 'number' || type === 'boolean') {
                 return String(v);
             } else if (v instanceof Color) {
                 return v.toString();

src/style-spec/reference/v8.json

@@ -2194,7 +2194,7 @@
         }
       },
       "to-string": {
-        "doc": "Converts the input value to a string. If the input is `null`, the result is `\"null\"`. If the input is a boolean, the result is `\"true\"` or `\"false\"`. If the input is a number, it is converted to a string as specified by the [\"NumberToString\" algorithm](https://tc39.github.io/ecma262/#sec-tostring-applied-to-the-number-type) of the ECMAScript Language Specification. If the input is a color, it is converted to a string of the form `\"rgba(r,g,b,a)\"`, where `r`, `g`, and `b` are numerals ranging from 0 to 255, and `a` ranges from 0 to 1. Otherwise, the input is converted to a string in the format specified by the [`JSON.stringify`](https://tc39.github.io/ecma262/#sec-json.stringify) function of the ECMAScript Language Specification.",
+        "doc": "Converts the input value to a string. If the input is `null`, the result is `\"\"`. If the input is a boolean, the result is `\"true\"` or `\"false\"`. If the input is a number, it is converted to a string as specified by the [\"NumberToString\" algorithm](https://tc39.github.io/ecma262/#sec-tostring-applied-to-the-number-type) of the ECMAScript Language Specification. If the input is a color, it is converted to a string of the form `\"rgba(r,g,b,a)\"`, where `r`, `g`, and `b` are numerals ranging from 0 to 255, and `a` ranges from 0 to 1. Otherwise, the input is converted to a string in the format specified by the [`JSON.stringify`](https://tc39.github.io/ecma262/#sec-json.stringify) function of the ECMAScript Language Specification.",
         "group": "Types",
         "sdk-support": {
           "basic functionality": {

test/integration/expression-tests/to-string/basic/test.json

@@ -15,7 +15,7 @@
       "isZoomConstant": true,
       "type": "string"
     },
-    "outputs": ["1", "false", "null", "string", "[1,2]", "{\"y\":1}"],
+    "outputs": ["1", "false", "", "string", "[1,2]", "{\"y\":1}"],
     "serialized": ["to-string", ["get", "x"]]
   }
 }