Test failure with mapnik 3.0.13

[tomhughes]

Running the 3.0.x branch with mapnik 3.0.13 is giving me a new test failure that wasn't present with 3.0.12:

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ERROR: python_tests.geometry_io_test.test_handling_geojson_null_geoms
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Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/home/thh/rpm/BUILD/python-mapnik-8139e5c7aebc2b555f991fc32f6190a1db1cf405/test/python_tests/geometry_io_test.py", line 335, in test_handling_geojson_null_geoms
    out_json = mapnik.Feature.from_geojson(j, ctx).to_geojson()
RuntimeError: Failed to parse geojson feature

[tomhughes]

It's the last entry in geojson_nulls that is triggering it:

    '{ "type": "Feature", "properties": { }, "geometry": { "type": "MultiPolygon", "coordinates": [ [ [ [] ] ] ] }}',

Note that it might not be the mapnik version change triggering it as other things have also change (boost from 1.60.0 to 1.63.0 to start with).

[springmeyer]

This should be solved now in the v3.0.x branch after #144