fix: allow content libraries to be created through Django admin (#31719)

Previously, CMS Django admin would demand that at least one
LTIConfig be associated with the new library. This is not
necessary, but Django thought it was, because the `blank`
flag was not enabled on the ContentLibrary model's
`authored_lti_config` many-to-many field.

This should not affect the experimental blockstore-based
library authoring frontend, which already allows libraries
to be created without any authorized_lti_configs.

This will not affect the existing modulestore-based
library authoring frontend, since it doesn't even use
any of the models in question.

A migration is included with this commit, but it should NOT
change MySQL schema, since `blank` is a Django-level validation
flag.

openedx/core/djangoapps/content_libraries/migrations/0009_alter_contentlibrary_authorized_lti_configs.py

@@ -0,0 +1,19 @@
+# Generated by Django 3.2.16 on 2023-02-06 21:39
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('lti1p3_tool_config', '0001_initial'),
+        ('content_libraries', '0008_auto_20210818_2148'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='contentlibrary',
+            name='authorized_lti_configs',
+            field=models.ManyToManyField(blank=True, help_text="List of authorized LTI tool configurations that can access this library's content through LTI launches, if empty no LTI launch is allowed.", related_name='content_libraries', to='lti1p3_tool_config.LtiTool'),
+        ),
+    ]

openedx/core/djangoapps/content_libraries/models.py

@@ -126,7 +126,8 @@ class ContentLibrary(models.Model):
         related_name='content_libraries',
         help_text=("List of authorized LTI tool configurations that can access "
                    "this library's content through LTI launches, if empty no LTI "
-                   "launch is allowed.")
+                   "launch is allowed."),
+        blank=True,
     )
 
     class Meta: