feat(overlay): Make stops overlay an empty FeatureGroup when not rendered

Author: David Emory

lib/components/map/stops-overlay.js

@@ -52,7 +52,7 @@ class StopsOverlay extends MapLayer {
     const { minZoom, queryMode, setLocation, stops } = this.props
 
     // don't render if below zoom threshold or transit not currently selected
-    if (this.context.map.getZoom() < minZoom || !hasTransit(queryMode)) return null
+    if (this.context.map.getZoom() < minZoom || !hasTransit(queryMode)) return <FeatureGroup />
 
     return (
       <FeatureGroup>