Do not treat empty payload as empty dict

This allows views to distinguish missing payload from empty payload.

Related: #3647, #4566

rest_framework/request.py

@@ -334,7 +334,7 @@ class Request:
             if media_type and is_form_media_type(media_type):
                 empty_data = QueryDict('', encoding=self._request._encoding)
             else:
-                empty_data = {}
+                empty_data = None
             empty_files = MultiValueDict()
             return (empty_data, empty_files)
 

tests/test_request.py

@@ -56,14 +56,14 @@ class TestContentParsing(TestCase):
         Ensure request.data returns empty QueryDict for GET request.
         """
         request = Request(factory.get('/'))
-        assert request.data == {}
+        assert request.data is None
 
     def test_standard_behaviour_determines_no_content_HEAD(self):
         """
         Ensure request.data returns empty QueryDict for HEAD request.
         """
         request = Request(factory.head('/'))
-        assert request.data == {}
+        assert request.data is None
 
     def test_request_DATA_with_form_content(self):
         """

tests/test_testing.py

@@ -5,7 +5,7 @@ from django.contrib.auth.models import User
 from django.shortcuts import redirect
 from django.test import TestCase, override_settings
 
-from rest_framework import fields, serializers
+from rest_framework import serializers
 from rest_framework.decorators import api_view
 from rest_framework.response import Response
 from rest_framework.test import (
@@ -35,13 +35,10 @@ def redirect_view(request):
     return redirect('/view/')
 
 
-class BasicSerializer(serializers.Serializer):
-    flag = fields.BooleanField(default=lambda: True)
-
-
 @api_view(['POST'])
 def post_view(request):
-    serializer = BasicSerializer(data=request.data)
+    serializer = serializers.Serializer(data=request.data)
+    serializer.allow_null = True
     serializer.is_valid(raise_exception=True)
     return Response(serializer.validated_data)
 
@@ -198,7 +195,7 @@ class TestAPITestClient(TestCase):
             content_type='application/json'
         )
         assert response.status_code == 200
-        assert response.data == {"flag": True}
+        assert response.data is None
 
 
 class TestAPIRequestFactory(TestCase):