Undefined proj4 string

Callback fixed and now handles projection files that return an `undefined` proj4 string from srs.parse, even after prepending `ESRI::`. Though still no test case for esri:: variant files that [succeed in this logic](https://github.com/mapbox/mapnik-omnivore/blob/esri-proj/lib/datasource-processor.js#L284-L290).

cc @willwhite