Fix for StockItem model

- Allow sales_order_line to be blank

InvenTree/stock/migrations/0030_auto_20200422_0015.py

@@ -0,0 +1,20 @@
+# Generated by Django 3.0.5 on 2020-04-22 00:15
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('order', '0023_auto_20200420_2309'),
+        ('stock', '0029_auto_20200421_2359'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='stockitem',
+            name='sales_order_line',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='stock_items', to='order.SalesOrderLineItem'),
+        ),
+    ]