Update SupplierPart model to allow data importing

- django-import-export plugin seems to require null=True for char fields
- yes, this goes against django "best practice"
- Hopefully a better solution can be found

InvenTree/company/migrations/0031_auto_20210103_2215.py

@@ -0,0 +1,61 @@
+# Generated by Django 3.0.7 on 2021-01-03 11:15
+
+import InvenTree.fields
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('part', '0060_merge_20201112_1722'),
+        ('company', '0030_auto_20201112_1112'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='supplierpart',
+            name='MPN',
+            field=models.CharField(blank=True, help_text='Manufacturer part number', max_length=100, null=True, verbose_name='MPN'),
+        ),
+        migrations.AlterField(
+            model_name='supplierpart',
+            name='SKU',
+            field=models.CharField(help_text='Supplier stock keeping unit', max_length=100, verbose_name='SKU'),
+        ),
+        migrations.AlterField(
+            model_name='supplierpart',
+            name='description',
+            field=models.CharField(blank=True, help_text='Supplier part description', max_length=250, null=True, verbose_name='Description'),
+        ),
+        migrations.AlterField(
+            model_name='supplierpart',
+            name='link',
+            field=InvenTree.fields.InvenTreeURLField(blank=True, help_text='URL for external supplier part link', null=True, verbose_name='Link'),
+        ),
+        migrations.AlterField(
+            model_name='supplierpart',
+            name='manufacturer',
+            field=models.ForeignKey(blank=True, help_text='Select manufacturer', limit_choices_to={'is_manufacturer': True}, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='manufactured_parts', to='company.Company', verbose_name='Manufacturer'),
+        ),
+        migrations.AlterField(
+            model_name='supplierpart',
+            name='note',
+            field=models.CharField(blank=True, help_text='Notes', max_length=100, null=True, verbose_name='Note'),
+        ),
+        migrations.AlterField(
+            model_name='supplierpart',
+            name='packaging',
+            field=models.CharField(blank=True, help_text='Part packaging', max_length=50, null=True),
+        ),
+        migrations.AlterField(
+            model_name='supplierpart',
+            name='part',
+            field=models.ForeignKey(help_text='Select part', limit_choices_to={'purchaseable': True}, on_delete=django.db.models.deletion.CASCADE, related_name='supplier_parts', to='part.Part', verbose_name='Base Part'),
+        ),
+        migrations.AlterField(
+            model_name='supplierpart',
+            name='supplier',
+            field=models.ForeignKey(help_text='Select supplier', limit_choices_to={'is_supplier': True}, on_delete=django.db.models.deletion.CASCADE, related_name='supplied_parts', to='company.Company', verbose_name='Supplier'),
+        ),
+    ]