Fix DictField.get_value to return empty

Without this, an empty field is not considered to be empty and then
validators might fail, e.g.
django.contrib.postgres.validators.RangeMinValueValidator with

> E   TypeError: '<' not supported between instances of 'NoneType' and 'int'

rest_framework/fields.py

@@ -1708,10 +1708,17 @@ class DictField(Field):
         self.child.bind(field_name='', parent=self)
 
     def get_value(self, dictionary):
+        if self.field_name not in dictionary:
+            if getattr(self.root, 'partial', False):
+                return empty
         # We override the default field access in order to support
         # dictionaries in HTML forms.
         if html.is_html_input(dictionary):
-            return html.parse_html_dict(dictionary, prefix=self.field_name)
+            val = dictionary.getlist(self.field_name, {})
+            if len(val) > 0:
+                # Support QueryDict lists in HTML input.
+                return val
+            return html.parse_html_list(dictionary, prefix=self.field_name, default=empty)
         return dictionary.get(self.field_name, empty)
 
     def to_internal_value(self, data):

tests/test_fields.py

@@ -16,7 +16,7 @@ from django.utils.timezone import activate, deactivate, override, utc
 import rest_framework
 from rest_framework import exceptions, serializers
 from rest_framework.compat import ProhibitNullCharactersValidator
-from rest_framework.fields import DjangoImageField, is_simple_callable
+from rest_framework.fields import DjangoImageField, empty, is_simple_callable
 
 try:
     import typings
@@ -168,6 +168,11 @@ class TestEmpty:
         output = field.run_validation()
         assert output is 123
 
+    def test_dictfield_empty(self):
+        field = serializers.DictField()
+        field.field_name = 'dict_field'
+        assert field.get_value(QueryDict()) is empty
+
 
 class TestSource:
     def test_source(self):