FIX: Handle missing sketch logo during backup restore (#36541)

When restoring an empty backup, 'SiteIconManager.ensure_optimized!' is
triggered during 'reload_site_settings', before 'run_seed_fu' has
created the sketch logo upload (ID -6). This causes
'ActiveRecord::RecordNotFound'.

Use 'find_by' instead of 'find' to gracefully return 'nil' when the
sketch logo doesn't exist yet.

Internal ref - t/8308

lib/site_icon_manager.rb

@@ -123,7 +123,7 @@ module SiteIconManager
       value = SiteSetting.get(setting_name)
       return value if value
     end
-    return Upload.find(SKETCH_LOGO_ID) if info[:fallback_to_sketch]
+    return Upload.find_by(id: SKETCH_LOGO_ID) if info[:fallback_to_sketch]
     nil
   end
 end

spec/lib/site_icon_manager_spec.rb

@@ -24,7 +24,21 @@ RSpec.describe SiteIconManager do
     # Falls back to sketch for some icons
     expect(SiteIconManager.favicon.upload_id).to eq(SiteIconManager::SKETCH_LOGO_ID)
     expect(SiteIconManager.mobile_logo).to eq(nil)
+  end
 
+  it "handles missing sketch logo gracefully" do
+    SiteSetting.logo = ""
+    SiteSetting.logo_small = ""
+
+    Upload.find_by(id: SiteIconManager::SKETCH_LOGO_ID)&.destroy
+
+    expect(SiteIconManager.favicon).to eq(nil)
+    expect(SiteIconManager.large_icon).to eq(nil)
+    expect(SiteIconManager.mobile_logo).to eq(nil)
+    expect(SiteIconManager.opengraph_image).to eq(nil)
+  end
+
+  it "resizes icons correctly" do
     SiteSetting.logo_small = upload
 
     # Always resizes to 512x512