Check that action.mapId exists

I'm running into an error where an action is being processed through `mapMiddleware` that does not have a `mapId`, throwing an error at the following line: https://github.com/mapbox/mapbox-gl-redux/blob/99fcb2b6f43c60d02c36cae80d2ca955c8879aac/src/index.js#L157

We should check that `action.mapId` exists before looking for its value in the object.

Start in src/index.js at the mapMiddleware code around line 157 and trace how action.mapId is used to select a map. Done means actions without mapId no longer cause the lookup error while actions with mapId continue to be processed normally.

Written by the indexing model from the issue text.