Enable blank values for safe-app provider (#115)

- SafeApp.provider can now be blank
- The blank field is used for validation when creating an instance for the model whereas null is only used for the database (see: https://docs.djangoproject.com/en/dev/ref/models/fields/#blank)

src/safe_apps/migrations/0003_alter_safeapp_provider.py

@@ -0,0 +1,24 @@
+# Generated by Django 3.2.4 on 2021-06-18 12:28
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("safe_apps", "0002_rename_networks_safeapp_chain_ids"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="safeapp",
+            name="provider",
+            field=models.ForeignKey(
+                blank=True,
+                null=True,
+                on_delete=django.db.models.deletion.SET_NULL,
+                to="safe_apps.provider",
+            ),
+        ),
+    ]

src/safe_apps/models.py

@@ -16,7 +16,9 @@ class SafeApp(models.Model):
     icon_url = models.URLField()
     description = models.CharField(max_length=200)
     chain_ids = ArrayField(models.IntegerField())
-    provider = models.ForeignKey(Provider, null=True, on_delete=models.SET_NULL)
+    provider = models.ForeignKey(
+        Provider, null=True, blank=True, on_delete=models.SET_NULL
+    )
 
     def __str__(self):
         return f"{self.name} | {self.url} | chain_ids={self.chain_ids}"

src/safe_apps/tests/test_models.py

@@ -16,3 +16,10 @@ def test_str_method_outputs_name_url_chain_id(self):
             str(safe_app),
             f"{safe_app.name} | {safe_app.url} | chain_ids={safe_app.chain_ids}",
         )
+
+    @staticmethod
+    def test_empty_provider_validation():
+        safe_app = SafeAppFactory.create(provider=None)
+
+        # Run validations including blank checks
+        safe_app.full_clean()