Make discount URL field non-nullable

This changes the field to be in line with Django recommendations:

> Avoid using null on string-based fields such as `CharField` and
> `TextField.` If a string-based field has `null=True`, that means it
> has two possible values for "no data": `NULL`, and the empty string.
> In most cases, it's redundant to have two possible values for "no
> data;" the Django convention is to use the empty string, not `NULL`.

https://docs.djangoproject.com/en/3.2/ref/models/fields/#django.db.models.Field.null

Three more fields on `Trip` have a large number of records with `null`
-- I'd need to address those before I can turn on linting for DJ001.

ws/migrations/0055_discount_url_non_null.py

@@ -0,0 +1,19 @@
+# Generated by Django 3.2.19 on 2023-06-18 16:24
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('ws', '0054_allow_null_reminder_sent_at'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='discount',
+            name='url',
+            field=models.URLField(blank=True, default=''),
+            preserve_default=False,
+        ),
+    ]

ws/models.py

@@ -129,7 +129,7 @@ class Discount(models.Model):
     name = models.CharField(max_length=255)
     summary = models.CharField(max_length=255)
     terms = models.TextField(max_length=4095)
-    url = models.URLField(null=True, blank=True)
+    url = models.URLField(blank=True)
     ga_key = models.CharField(
         max_length=63,
         # If blank, then we don't actually report this information to a spreadsheet