CSS - control group box shadow still visible when empty

[Duder-onomy]

Hi!

When a maps control group is empty, you can still see the control groups box shadow:

mapbox-gl-js version: #master

browser: All

Steps to Trigger Behavior

1. Attempt to render the 'geolocation' control
2. Block geolocation access in your browser.
   In Chrome:
   Go to settings -> advanced -> location Add the current URL to your block list.

Expected Behavior

The selector .mapboxgl-ctrl-group should not render its box-shadow when it has no children.

Actual Behavior

It renders its box shadow always.

---

I think a simple fix for this would be to use the :not(:empty) selector when applying the box shadow to the control-group.