feat: make User.full_name field non-nullable

This commit makes the User.full_name field non-nullable to ensure that there is only one representation of the empty value - the empty string. Currently, empty values for this field can be either None or the empty string. Because we're transitioning from a nullable to non-nullable state, a default is required to handle existing rows with a null value.

This is best practice; Django discourages setting both null=True and blank=True on CharField model fields. Furthermore, this was required by our event bus work. If an empty value is represented by None, this causes issues with the event bus, because None is not JSON serializable. Instead of converting a None value to the empty string in the event producer, correcting the the model definition is a better approach.

edx_exams/apps/core/migrations/0019_alter_user_full_name.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.2.21 on 2023-10-02 20:21
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0018_staff_roles'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='user',
+            name='full_name',
+            field=models.CharField(blank=True, default='', max_length=255, verbose_name='Full Name'),
+        ),
+    ]

edx_exams/apps/core/models.py

@@ -24,7 +24,8 @@ class User(AbstractUser):
     .. pii_retirement: local_api
 
     """
-    full_name = models.CharField(_('Full Name'), max_length=255, blank=True, null=True)
+    # The default empty string was added to change full_name from nullable to non-nullable.
+    full_name = models.CharField(_('Full Name'), max_length=255, blank=True, default='')
 
     lms_user_id = models.IntegerField(null=True, db_index=True)