Drop use of deprecated NullBooleanField.

src/oscar/apps/catalogue/abstract_models.py

@@ -1040,7 +1040,7 @@ class AbstractProductAttributeValue(models.Model):
 
     value_text = models.TextField(_('Text'), blank=True, null=True)
     value_integer = models.IntegerField(_('Integer'), blank=True, null=True, db_index=True)
-    value_boolean = models.NullBooleanField(_('Boolean'), blank=True, db_index=True)
+    value_boolean = models.BooleanField(_('Boolean'), blank=True, null=True, db_index=True)
     value_float = models.FloatField(_('Float'), blank=True, null=True, db_index=True)
     value_richtext = models.TextField(_('Richtext'), blank=True, null=True)
     value_date = models.DateField(_('Date'), blank=True, null=True, db_index=True)

src/oscar/apps/catalogue/migrations/0022_auto_20210210_0539.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.1.6 on 2021-02-10 05:39
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('catalogue', '0021_auto_20201005_0844'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='productattributevalue',
+            name='value_boolean',
+            field=models.BooleanField(blank=True, db_index=True, null=True, verbose_name='Boolean'),
+        ),
+    ]