Show the Mapbox logo when no style is provided

[ryanhamley]

Launch Checklist

• briefly describe the changes in this PR
  • Show Mapbox logo if no style is provided
• include before/after visuals or gifs if this PR includes visual changes
  • Before: https://jsbin.com/suduqipoba/edit?html,output
  • After: https://jsbin.com/yesixowabe/edit?html,output
• write tests for all new functionality
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Show Mapbox logo if not style is provided for the map</changelog>

[asheemmamoowala]

Good to merge pending a unit/other test.