Teamsnap Availability Model fixes.

Accept null and blank for availability status code. Add the type property.
and migrations

teamsnap/migrations/0009_alter_availability_status_code.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.2.6 on 2021-12-29 16:24
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('teamsnap', '0008_alter_availability_status_code'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='availability',
+            name='status_code',
+            field=models.SmallIntegerField(blank=True, choices=[(1, 'Yes'), (0, 'No'), (2, 'Maybe'), (None, 'Unknown')], default=None, null=True),
+        ),
+    ]