Allow null values for StockItem.updated

- Just to make the tests run better

InvenTree/stock/migrations/0016_auto_20190512_2119.py

@@ -0,0 +1,18 @@
+# Generated by Django 2.2 on 2019-05-12 11:19
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stock', '0015_stockitem_delete_on_deplete'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='stockitem',
+            name='updated',
+            field=models.DateField(auto_now=True, null=True),
+        ),
+    ]