Fix support for ListField default values in HTML mode

The ListField handles html forms data differently than json data.  This section
would always return a value (an empty list) for a list field, even when that
field name was not posted at all.  This change checks for the field being
present and returns `empty` if it is not; this lets the standard default-value and
required field handling take over.

rest_framework/fields.py

@@ -1609,6 +1609,8 @@ class ListField(Field):
         # We override the default field access in order to support
         # lists in HTML forms.
         if html.is_html_input(dictionary):
+            if self.field_name not in dictionary:
+                return empty
             val = dictionary.getlist(self.field_name, [])
             if len(val) > 0:
                 # Support QueryDict lists in HTML input.

tests/test_fields.py

@@ -459,6 +459,31 @@ class TestHTMLInput:
         assert serializer.is_valid()
         assert serializer.validated_data == {'scores': [1]}
 
+    def test_querydict_list_input_no_values_uses_default(self):
+        """
+        When there are no values passed in, and default is set
+        The field should return the default value
+        """
+        class TestSerializer(serializers.Serializer):
+            a = serializers.IntegerField(required=True)
+            scores = serializers.ListField(default=lambda: [1, 3])
+
+        serializer = TestSerializer(data=QueryDict('a=1&'))
+        assert serializer.is_valid()
+        assert serializer.validated_data == {'a': 1, 'scores': [1, 3]}
+
+    def test_querydict_list_input_no_values_no_default_and_not_required(self):
+        """
+        When there are no keys passed, there is no default, and required=False
+        The field should be skipped
+        """
+        class TestSerializer(serializers.Serializer):
+            scores = serializers.ListField(required=False)
+
+        serializer = TestSerializer(data=QueryDict(''))
+        assert serializer.is_valid()
+        assert serializer.validated_data == {}
+
 
 class TestCreateOnlyDefault:
     def setup(self):