Change nullable check to resolve FutureWarning

[iamsaywhat]

Resolves several more cases of this warning in addition to #881, #595

FutureWarning: The behavior of this method will change in future versions. Use specific 'len(elem)' or 'elem is not None' test instead.

[coveralls]

coverage: 58.956%. remained the same
when pulling 679ba04 on iamsaywhat:master
into 1b7e3bd on geopython:master.