FIX: Allow deleting avatars from the selectable avatars setting (#26720)

Fixes two issues:

- frontend was reloading the page when clicking-to-remove avatar
- backend wasn't allowing resetting the setting by deleting all avatars

spec/requests/admin/site_settings_controller_spec.rb

@@ -269,6 +269,13 @@ RSpec.describe Admin::SiteSettingsController do
         expect(SiteSetting.title).to eq("")
       end
 
+      it "allows value to be a blank string for selectable_avatars" do
+        SiteSetting.selectable_avatars = [Fabricate(:image_upload)]
+        put "/admin/site_settings/selectable_avatars.json", params: { selectable_avatars: "" }
+        expect(response.status).to eq(200)
+        expect(SiteSetting.selectable_avatars).to eq([])
+      end
+
       it "sanitizes integer values" do
         put "/admin/site_settings/suggested_topics.json", params: { suggested_topics: "1,000" }