Fix #65 -- Disallow None value for product default prices (#3847)

* Fix #65 -- Disallow None value for product default prices

* Fix #65 -- Disallow None value for product default prices

* Rebase migration

src/pretix/base/migrations/0257_item_default_price_not_null.py

@@ -0,0 +1,19 @@
+# Generated by Django 4.2.9 on 2024-01-30 11:11
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("pretixbase", "0256_itemvariation_unavail_modes"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="item",
+            name="default_price",
+            field=models.DecimalField(decimal_places=2, default=0, max_digits=13),
+            preserve_default=False,
+        ),
+    ]