Part: Alter model fields to fix data import issues

Without setting 'null=True' on fields which may be empty,
the import subsequently fails if these fields are indeed empty

InvenTree/part/migrations/0048_auto_20200902_1404.py

@@ -0,0 +1,48 @@
+# Generated by Django 3.0.7 on 2020-09-02 14:04
+
+import InvenTree.fields
+import InvenTree.validators
+
+import markdownx
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('part', '0047_auto_20200808_0715'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='part',
+            name='IPN',
+            field=models.CharField(blank=True, help_text='Internal Part Number', max_length=100, null=True, validators=[InvenTree.validators.validate_part_ipn]),
+        ),
+        migrations.AlterField(
+            model_name='part',
+            name='keywords',
+            field=models.CharField(blank=True, help_text='Part keywords to improve visibility in search results', max_length=250, null=True),
+        ),
+        migrations.AlterField(
+            model_name='part',
+            name='link',
+            field=InvenTree.fields.InvenTreeURLField(blank=True, help_text='Link to extenal URL', null=True),
+        ),
+        migrations.AlterField(
+            model_name='part',
+            name='revision',
+            field=models.CharField(blank=True, help_text='Part revision or version number', max_length=100, null=True),
+        ),
+        migrations.AlterField(
+            model_name='part',
+            name='notes',
+            field=markdownx.models.MarkdownxField(blank=True, help_text='Part notes - supports Markdown formatting', null=True),
+        ),
+        migrations.AlterField(
+            model_name='part',
+            name='units',
+            field=models.CharField(blank=True, default='', help_text='Stock keeping units for this part', max_length=20, null=True),
+        ),
+    ]