Accept undefined in setFilter

This makes `setFilter` consistent with `setLayoutProperty` and
`setPaintProperty`, both of which accept either `null` or `undefined` to
unset the property value.

Helps diffing/setState, because in order for the input to setState
to be valid style JSON, we can't have `filter: null`, but rather need to
omit the `filter` property.  Without having `setFilter` accept `null`,
we'd have to (hackily) replace `undefined` with `null` in the style
diff results

js/style/style.js

@@ -521,7 +521,7 @@ class Style extends Evented {
 
         const layer = this.getLayer(layerId);
 
-        if (filter !== null && this._validate(validateStyle.filter, `layers.${layer.id}.filter`, filter)) return;
+        if (filter !== null && filter !== undefined && this._validate(validateStyle.filter, `layers.${layer.id}.filter`, filter)) return;
 
         if (util.deepEqual(layer.filter, filter)) return;
         layer.filter = util.clone(filter);