Need to check if container option is provided

[jingsam]

Recently I am reviewing the source code. As I know, the options.container must be provided as string or HTMLElement. I think bellow code would fail to validate container if it is undefined or other types.

https://github.com/mapbox/mapbox-gl-js/blob/master/src/ui/map.js#L280-L289

        if (typeof options.container === 'string') {
            const container = window.document.getElementById(options.container);
            if (!container) {
                throw new Error(`Container '${options.container}' not found.`);
            } else {
                this._container = container;
            }
        } else {
            this._container = options.container;
        }