Check CSV for empty strings

[MuellerMatthew]

You were previously checking if parsed[i][lonfield] was undefined twice within the same if statement. I removed the duplicated check, and added a check for empty strings which I think is what was originally intended for the second part of the if statement.

[tmcw]

This part of leaflet-omnivore is in a built file. Please fix this upstream in csv2geojson instead of in the built file.