many methods error after `map.setStyle(null)`.

Nobody has claimed this yet.

**mapbox-gl-js version**: master

**browser**: all

### Steps to Trigger Behavior

1. `map.setStyle(null)`
2. call a method such as `map.getLayer("asdf")`

### Link to Demonstration

https://jsbin.com/jufijesasu/edit?html,output

### Expected Behavior

`Uncaught TypeError: Cannot read property 'getLayer' of undefined`

### Actual Behavior

Returns nothing and *maybe* fires an error event.

---

I'm seeing two possible approaches to fixing this.

A) Never let a user set the style to null (`map.setStyle(null)`).
B) handle `undefined` wherever `map.style` is accessed

Flow highlights all the cases when you make `map.style` optional.

```diff
diff --git a/src/ui/map.js b/src/ui/map.js
index 8ff6957ac..452880d29 100755
--- a/src/ui/map.js
+++ b/src/ui/map.js
@@ -228,7 +228,7 @@ const defaultOptions = {
* @see [Display a map](https://www.mapbox.com/mapbox-gl-js/examples/)
*/
class Map extends Camera {
- style: Style;
+ style: ?Style;
painter: Painter;

_container: HTMLElement;
```

Start with src/ui/map.js and reproduce the jsbin case by calling map.setStyle(null), then map.getLayer("asdf") and other affected methods. Trace the map.style accesses highlighted by the issue's Flow diff. Done means the behavior after a null style is consistent and the reported methods no longer fail silently or emit an unexpected error.

Written by the indexing model from the issue text.