Try a more localised fix to the data=None problem

rest_framework/fields.py

@@ -306,6 +306,7 @@ class WritableField(Field):
             return
 
         try:
+            data = data or {}
             if self.use_files:
                 files = files or {}
                 try:

rest_framework/serializers.py

@@ -156,7 +156,7 @@ class BaseSerializer(WritableField):
 
         self.context = context or {}
 
-        self.init_data = data or {}
+        self.init_data = data
         self.init_files = files
         self.object = instance
         self.fields = self.get_fields()