Fixed #28404 -- empty_value_display for fields with empty string values

PR recommendation by @felixxm
django#17702 (comment)

AUTHORS

@@ -47,6 +47,7 @@ answer newbie questions, and generally made Django that much better:
     Aleksandra Sendecka <asendecka@hauru.eu>
     Aleksi Häkli <aleksi.hakli@iki.fi>
     Alex Dutton <django@alexdutton.co.uk>
+    Alexander Lazarević <laza@e11bits.com>
     Alexander Myodov <alex@myodov.com>
     Alexandr Tatarinov <tatarinov.dev@gmail.com>
     Alex Aktsipetrov <alex.akts@gmail.com>

django/contrib/admin/templatetags/admin_list.py

@@ -179,7 +179,7 @@ def make_qs_param(t, n):
 
 def _boolean_icon(field_val):
     icon_url = static(
-        "admin/img/icon-%s.svg" % {True: "yes", False: "no", None: "unknown"}[field_val]
+        "admin/img/icon-%s.svg" % {True: "yes", False: "no"}.get(field_val, "unknown")
     )
     return format_html('<img src="{}" alt="{}">', icon_url, field_val)
 

django/contrib/admin/utils.py

@@ -433,7 +433,7 @@ def display_for_field(value, field, empty_value_display):
     # general null test.
     elif isinstance(field, models.BooleanField):
         return _boolean_icon(value)
-    elif value is None:
+    elif value in field.empty_values:
         return empty_value_display
     elif isinstance(field, models.DateTimeField):
         return formats.localize(timezone.template_localtime(value))

tests/admin_changelist/tests.py

@@ -253,6 +253,28 @@ def test_result_list_empty_changelist_value(self):
             "Failed to find expected row element: %s" % table_output,
         )
 
+    def test_result_list_empty_changelist_string_value(self):
+        new_child = Child.objects.create(name="", parent=None)
+        request = self.factory.get("/child/")
+        request.user = self.superuser
+        m = ChildAdmin(Child, custom_site)
+        cl = m.get_changelist_instance(request)
+        cl.formset = None
+        template = Template(
+            "{% load admin_list %}{% spaceless %}{% result_list cl %}{% endspaceless %}"
+        )
+        context = Context({"cl": cl, "opts": Child._meta})
+        table_output = template.render(context)
+        link = reverse("admin:admin_changelist_child_change", args=(new_child.id,))
+        row_html = build_tbody_html(
+            new_child, link, "-", '<td class="field-parent nowrap">-</td>'
+        )
+        self.assertNotEqual(
+            table_output.find(row_html),
+            -1,
+            "Failed to find expected row element: %s" % table_output,
+        )
+
     def test_result_list_set_empty_value_display_on_admin_site(self):
         """
         Empty value display can be set on AdminSite.

tests/admin_utils/tests.py

@@ -159,9 +159,12 @@ def test_empty_value_display_for_field(self):
             models.TimeField(),
         ]
         for model_field in tests:
-            with self.subTest(model_field=model_field):
-                display_value = display_for_field(None, model_field, self.empty_value)
-                self.assertEqual(display_value, self.empty_value)
+            for value in model_field.empty_values:
+                with self.subTest(model_field=model_field, empty_value=value):
+                    display_value = display_for_field(
+                        value, model_field, self.empty_value
+                    )
+                    self.assertEqual(display_value, self.empty_value)
 
     def test_empty_value_display_choices(self):
         model_field = models.CharField(choices=((None, "test_none"),))

tests/admin_widgets/tests.py

@@ -49,6 +49,7 @@
     UnsafeLimitChoicesTo,
     VideoStream,
 )
+from .widgetadmin import AlbumAdmin
 from .widgetadmin import site as widget_admin_site
 
 
@@ -639,9 +640,12 @@ def test_readonly_fields(self):
             html=True,
         )
         response = self.client.get(reverse("admin:admin_widgets_album_add"))
+        empty_value_display = AlbumAdmin(
+            Album, widget_admin_site
+        ).get_empty_value_display()
         self.assertContains(
             response,
-            '<div class="readonly"></div>',
+            '<div class="readonly">%s</div>' % empty_value_display,
             html=True,
         )