models: Update can_remove_subscribers_group field to not be null.

This commit udpates can_remove_subscribers_group to be not null.
We already added a migration to set the value of this field for
existing streams and also added a commit to set this field to
admins system group for now while creating streams.

zerver/migrations/0410_alter_stream_can_remove_subscribers_group.py

@@ -0,0 +1,21 @@
+# Generated by Django 4.0.6 on 2022-07-22 14:47
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("zerver", "0409_set_default_for_can_remove_subscribers_group"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="stream",
+            name="can_remove_subscribers_group",
+            field=models.ForeignKey(
+                on_delete=django.db.models.deletion.RESTRICT, to="zerver.usergroup"
+            ),
+        ),
+    ]

zerver/models.py

@@ -2511,8 +2511,8 @@ class Stream(models.Model):
     # deleting a user group in case it is referenced by this settig.
     # We are not using PROTECT since we want to allow deletion of user groups
     # when realm itself is deleted.
-    can_remove_subscribers_group: Optional[UserGroup] = models.ForeignKey(
+    can_remove_subscribers_group: UserGroup = models.ForeignKey(
-        UserGroup, null=True, on_delete=models.RESTRICT
+        UserGroup, on_delete=models.RESTRICT
     )
 
     # The very first message ID in the stream.  Used to help clients