Return rather than fail if resolution is not defined

When using a source which has to parse data, it is possible for the source to `change` before the map has called `loadFeatures`, triggering the assertion in debug mode and failing entirely otherwise.  This proposes to remove the assertion and simply return if `resolution_` is not defined.

An alternate approach I considered was to delay registering for the `change` event on the origin source until the first time loadFeatures is called, but this seemed simpler and safer.