Revert "Non-required fields with 'allow_null=True' should not imply a default value (#5639)"

This reverts commit 905a5579df.

Closes #5708

rest_framework/fields.py

@@ -442,10 +442,10 @@ class Field(object):
         except (KeyError, AttributeError) as exc:
             if self.default is not empty:
                 return self.get_default()
-            if not self.required:
-                raise SkipField()
             if self.allow_null:
                 return None
+            if not self.required:
+                raise SkipField()
             msg = (
                 'Got {exc_type} when attempting to get a value for field '
                 '`{field}` on serializer `{serializer}`.\nThe serializer '

tests/test_serializer.py

@@ -384,14 +384,6 @@ class TestNotRequiredOutput:
         serializer.save()
         assert serializer.data == {'included': 'abc'}
 
-    def test_not_required_output_for_allow_null_field(self):
-        class ExampleSerializer(serializers.Serializer):
-            omitted = serializers.CharField(required=False, allow_null=True)
-            included = serializers.CharField()
-
-        serializer = ExampleSerializer({'included': 'abc'})
-        assert 'omitted' not in serializer.data
-
 
 class TestDefaultOutput:
     def setup(self):