Fix evented.listens to return false when no listener exists

[ryanhamley]

Launch Checklist

• briefly describe the changes in this PR
  • This picks up Evented.prototype.listen(type) method return undefined #9985 which has no movement since September and no CLA signed
  • evented.listens is documented as returning false when no listener is available. instead it returns undefined. this ensures that false is always returned while also cleaning up some typos in the doc string
• write tests for all new functionality
• document any changes to public APIs
  • fixes listens so that it matches current documentation
• 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>Ensure evented.listens returns 'false' when no listener is available</changelog>