Support empty file fields

rest_framework/fields.py

@@ -913,6 +913,8 @@ class FileField(Field):
 
     def to_representation(self, value):
         if self.use_url:
+            if not value:
+                return None
             url = settings.MEDIA_URL + value.url
             request = self.context.get('request', None)
             if request is not None:

tests/test_fields.py

@@ -879,7 +879,8 @@ class TestFileField(FieldValues):
         (MockFile(name='x' * 100, size=10), ['Ensure this filename has at most 10 characters (it has 100).'])
     ]
     outputs = [
-        (MockFile(name='example.txt', url='/example.txt'), '/example.txt')
+        (MockFile(name='example.txt', url='/example.txt'), '/example.txt'),
+        ('', None)
     ]
     field = fields.FileField(max_length=10)