Returning None from filter_layer in translation throws error

[kinimesi]

According to the docs, filter_layer can return a layer or None. However, returning None throws the following error:

ogr2osm/ogr2osm/ogr_datasource.py", line 101, in __get_source_reprojection_func
    layer_spatial_ref = layer.GetSpatialRef()
AttributeError: 'NoneType' object has no attribute 'GetSpatialRef'

Looking into the original library, seems like there was a check for None, but it got lost during refactoring.

[roelderickx]

Good catch!
The original check is still in the refactored code:

ogr2osm/ogr2osm/osm_data.py

Line 418 in 70d2f04

if layer:

But that code is only executed after we tried to fetch the projection, which is of course impossible for a None layer.
I'll merge the pull request.