Admin: allow blank field from admin (#11962)

The field allows null, but we don't allow blank in the admin.

readthedocs/projects/migrations/0144_addons_blank_field.py

@@ -0,0 +1,22 @@
+# Generated by Django 4.2.17 on 2025-01-29 15:02
+
+from django.db import migrations, models
+import django.db.models.deletion
+from django_safemigrate import Safe
+
+
+class Migration(migrations.Migration):
+    safe = Safe.always
+
+    dependencies = [
+        ('builds', '0059_add_version_date_index'),
+        ('projects', '0143_addons_flyout_position'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='addonsconfig',
+            name='options_base_version',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, to='builds.version', verbose_name='Base version to compare against (eg. DocDiff, File Tree Diff)'),
+        ),
+    ]

readthedocs/projects/models.py

@@ -175,6 +175,7 @@ class AddonsConfig(TimeStampedModel):
         "builds.Version",
         verbose_name=_("Base version to compare against (eg. DocDiff, File Tree Diff)"),
         null=True,
+        blank=True,
         on_delete=models.SET_NULL,
     )