Make salvage value a required field on asset

Closes #439

assets/migrations/0024_alter_asset_salvage_value.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.2.12 on 2022-02-14 23:43
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('assets', '0023_alter_asset_purchased_from'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='asset',
+            name='salvage_value',
+            field=models.DecimalField(decimal_places=2, max_digits=10, null=True),
+        ),
+    ]