🩹(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
+        ),
+    ]

src/backend/core/tests/migrations/test_0018_update_blank_title.py

@@ -0,0 +1,35 @@
+import pytest
+
+from core import factories
+
+
+@pytest.mark.django_db
+def test_update_blank_title_migration(migrator):
+    """
+    Test that the migration fixes the titles of documents that are
+    "Untitled document", "Unbenanntes Dokument" or "Document sans titre"
+    """
+    migrator.apply_initial_migration(("core", "0017_add_fields_for_soft_delete"))
+
+    english_doc = factories.DocumentFactory(title="Untitled document")
+    german_doc = factories.DocumentFactory(title="Unbenanntes Dokument")
+    french_doc = factories.DocumentFactory(title="Document sans titre")
+    other_doc = factories.DocumentFactory(title="My document")
+
+    assert english_doc.title == "Untitled document"
+    assert german_doc.title == "Unbenanntes Dokument"
+    assert french_doc.title == "Document sans titre"
+    assert other_doc.title == "My document"
+
+    # Apply the migration
+    migrator.apply_tested_migration(("core", "0018_update_blank_title"))
+
+    english_doc.refresh_from_db()
+    german_doc.refresh_from_db()
+    french_doc.refresh_from_db()
+    other_doc.refresh_from_db()
+
+    assert english_doc.title == None
+    assert german_doc.title == None
+    assert french_doc.title == None
+    assert other_doc.title == "My document"