Fix serialization of properties holding None values

[nmandery]

The current implementation fails to serialize properties holding the value None. This may not affect al types of field as it depends how the field handles its conversion in its Field.to_representation() method. At least with datetime fields the attempt to serialize a None value will result in an AttributeError -- see the following stacktrace:

  [...]
  File "/home/nico/environments/venv-django17/local/lib/python2.7/site-packages/rest_framework/serializers.py", line 466, in data
    ret = super(Serializer, self).data
  File "/home/nico/environments/venv-django17/local/lib/python2.7/site-packages/rest_framework/serializers.py", line 213, in data
    self._data = self.to_representation(self.instance)
  File "/home/nico/environments/venv-django17/local/lib/python2.7/site-packages/rest_framework/serializers.py", line 435, in to_representation
    ret[field.field_name] = field.to_representation(attribute)
  File "/home/nico/environments/venv-django17/local/lib/python2.7/site-packages/rest_framework_gis/serializers.py", line 27, in to_representation
    ("features", super(GeoFeatureModelListSerializer, self).to_representation(data))
  File "/home/nico/environments/venv-django17/local/lib/python2.7/site-packages/rest_framework/serializers.py", line 568, in to_representation
    self.child.to_representation(item) for item in iterable
  File "/home/nico/environments/venv-django17/local/lib/python2.7/site-packages/rest_framework_gis/serializers.py", line 125, in to_representation
    feature["properties"] = self.get_properties(instance, fields)
  File "/home/nico/environments/venv-django17/local/lib/python2.7/site-packages/rest_framework_gis/serializers.py", line 148, in get_properties
    properties[field.field_name] = field.to_representation(value)
  File "/home/nico/environments/venv-django17/local/lib/python2.7/site-packages/rest_framework/fields.py", line 883, in to_representation
    return value.strftime(self.format)
AttributeError: 'NoneType' object has no attribute 'strftime'

This pull request addresses this issue by calling the to_representation method only when there is an actual value to convert.

[landscape-bot]

Code quality remained the same when pulling b02f8c7 on nmandery:feature/serialize-none-values into 73ac3c0 on djangonauts:master.

[nemesifier]

thx @nmandery