🩹(migration) add migration to update default titles

The frontend was setting a default titles for
documents with empty titles.
This migration updates the document table to set
the title to null instead of the default title.
We add a test to ensure that the migration
works as expected.

src/backend/core/migrations/0018_update_blank_title.py

@@ -0,0 +1,25 @@
+from django.db import migrations
+
+
+def update_titles_to_null(apps, schema_editor):
+    """
+    If the titles are "Untitled document" or "Unbenanntes Dokument" or "Document sans titre"
+    we set them to Null
+    """
+    Document = apps.get_model("core", "Document")
+    Document.objects.filter(
+        title__in=["Untitled document", "Unbenanntes Dokument", "Document sans titre"]
+    ).update(title=None)
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("core", "0017_add_fields_for_soft_delete"),
+    ]
+
+    operations = [
+        migrations.RunPython(
+            update_titles_to_null,
+            reverse_code=migrations.RunPython.noop
+        ),
+    ]