geotable.load fails for projecting when there are empty rows

example dataset
```csv
id,x,y,lat,lng,population
105477,39.4951389,-6.4759722,-6.4759722,39.4951389,4.79366
,,,,,
```

error
```
return geotable.load(self.source_geotable_path, target_proj4=utm_proj4)
File "/home/salah/.virtualenvs/env/lib/python3.7/site-packages/geotable/__init__.py", line 402, in load
**kw)
File "/home/salah/.virtualenvs/env/lib/python3.7/site-packages/geotable/__init__.py", line 85, in load
t = Class._load(source_path_or_url, source_proj4, target_proj4, **kw)
File "/home/salah/.virtualenvs/env/lib/python3.7/site-packages/geotable/__init__.py", line 113, in _load
source_path, source_proj4, target_proj4, **kw)
File "/home/salah/.virtualenvs/env/lib/python3.7/site-packages/geotable/__init__.py", line 186, in from_csv
geometry_objects.append(f(load_geometry_object(row)))
File "/home/salah/.virtualenvs/env/lib/python3.7/site-packages/geotable/projections.py", line 38, in transform_shapely_geometry
return wkb.loads(transform_gdal_geometry(gdal_geometry).ExportToWkb())
File "/home/salah/.virtualenvs/env/lib/python3.7/site-packages/geotable/projections.py", line 26, in transform_gdal_geometry
) % (source_proj4, target_proj4, gdal_geometry.ExportToWkt()))
geotable.exceptions.CoordinateTransformationError: coordinate transformation failed (source_proj4='+proj=longlat +datum=WGS84 +no_defs', target_proj4='+proj=utm +zone=37 +south +datum=WGS84 +units=m +no_defs', wkt='POINT EMPTY')
```