FIX: Use the default value correctly for theme settings of type uploads (#20541)

When a theme setting of type `upload` has a default upload, it should return the URL of the specified default upload until a custom upload is used for the setting. However, currently this isn't the case and we get null instead of the default upload URL.

The reason for this is because the `super` method of `#value` already returns the default upload URL (if there's one), so we can't pass that to `cdn_url` which expects an upload ID:

c961dcc757/lib/theme_settings_manager.rb (L212)

This commit fixes the bug by skipping the call to `cdn_url` when we fallback to the default upload for the setting value.

lib/theme_settings_manager.rb

@@ -184,7 +184,7 @@ class ThemeSettingsManager
 
   class Upload < self
     def value
-      cdn_url(super)
+      has_record? ? cdn_url(db_record.value) : default
     end
 
     def default

spec/fixtures/theme_settings/valid_settings.yaml

@@ -71,7 +71,7 @@ enum_setting_03:
 
 upload_setting:
   type: upload
-  default: ""
+  default: "default-upload"
 
 invalid_json_schema_setting:
   default: ""

spec/lib/theme_settings_manager_spec.rb

@@ -161,12 +161,30 @@ RSpec.describe ThemeSettingsManager do
       ).to be_truthy
     end
 
-    it "returns the CDN URL" do
-      upload_setting = find_by_name(:upload_setting)
-      upload_setting.value = upload.url
-      theme.reload
+    describe "#value" do
+      context "when it's changed to a custom upload" do
+        it "returns CDN URL" do
+          upload_setting = find_by_name(:upload_setting)
+          upload_setting.value = upload.url
+          theme.reload
 
-      expect(upload_setting.value).to eq(Discourse.store.cdn_url(upload.url))
+          expect(upload_setting.value).to eq(Discourse.store.cdn_url(upload.url))
+        end
+      end
+
+      context "when there's a default upload" do
+        it "returns CDN URL" do
+          theme.set_field(
+            target: :common,
+            name: "default-upload",
+            type: :theme_upload_var,
+            upload_id: upload.id,
+          )
+          theme.save!
+          upload_setting = find_by_name(:upload_setting)
+          expect(upload_setting.value).to eq(Discourse.store.cdn_url(upload.url))
+        end
+      end
     end
   end
 end