Return Empty when getting listserializers from empty html input

If a non-required nested list serializer is *not* included in
postdata from an HTML form, the field validation nonetheless returns
an empty list.

This means you can easily clobber existing data when PUTting to
an existing resource, just by not including some list fields.

This is made worse by DRF treating skipping request parsing for empty input.
- encode#3647 and encode#4566 are "fixed" but do not appear to fix the problem;
empty JSON posts still skip request parsing, and list fields not included in
the post still get set to empty lists.

rest_framework/serializers.py

@@ -578,7 +578,7 @@ def get_value(self, dictionary):
         # We override the default field access in order to support
         # lists in HTML forms.
         if html.is_html_input(dictionary):
-            return html.parse_html_list(dictionary, prefix=self.field_name)
+            return html.parse_html_list(dictionary, prefix=self.field_name) or empty
         return dictionary.get(self.field_name, empty)
 
     def run_validation(self, data=empty):

tests/test_serializer_lists.py

@@ -146,7 +146,7 @@ class TestNestedListSerializer:
 
     def setup(self):
         class TestSerializer(serializers.Serializer):
-            integers = serializers.ListSerializer(child=serializers.IntegerField())
+            integers = serializers.ListSerializer(child=serializers.IntegerField(), required=False)
             booleans = serializers.ListSerializer(child=serializers.BooleanField())
 
             def create(self, validated_data):
@@ -221,6 +221,22 @@ def test_validate_html_input(self):
         assert serializer.is_valid()
         assert serializer.validated_data == expected_output
 
+    def test_validate_empty_html_input(self):
+        """
+        When a field isn't present in HTML input, the field isn't included
+        in the output.
+        """
+        input_data = MultiValueDict({
+            "booleans[0]": ["true"],
+            "booleans[1]": ["false"]
+        })
+        expected_output = {
+            "booleans": [True, False]
+        }
+        serializer = self.Serializer(data=input_data)
+        assert serializer.is_valid()
+        assert serializer.validated_data == expected_output
+
 
 class TestNestedListOfListsSerializer:
     def setup(self):