fix: allow null for book category many to many

book/migrations/0006_alter_book_category.py

+# Generated by Django 5.0.7 on 2024-07-14 13:48
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('book', '0005_remove_book_category_book_category'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='book',
+            name='category',
+            field=models.ManyToManyField(blank=True, null=True, to='book.category'),
+        ),
+    ]