Check for None before calling to_representation

Django REST Framework also does this in serializers.

https://github.com/encode/django-rest-framework/issues/2299
https://github.com/encode/django-rest-framework/blob/4d57d46bf89f54f276909219ee06f0a2bf112f9f/rest_framework/serializers.py#L518

Fixes #203.

src/drf_yasg/utils.py

@@ -404,7 +404,7 @@ def get_field_default(field):
                                "called; 'default' will not be set on schema", field, exc_info=True)
                 default = serializers.empty
 
-        if default is not serializers.empty:
+        if default is not serializers.empty and default is not None:
             try:
                 default = field.to_representation(default)
                 # JSON roundtrip ensures that the value is valid JSON;