PERF: Eager load field causing N+1 issue (#27626)

* PERF: Fix N+1 issue for javascript_cache

* FIX: missing upload fields should still appear in stylesheets

Sass is still expected to compile successfully even without uploads.
Revert a blank upload to have a blank URL

* DEV: remove unneeded test comment

---------

Co-authored-by: Jeff Wong <awole20@gmail.com>

app/models/theme.rb

@@ -714,17 +714,21 @@ class Theme < ActiveRecord::Base
 
   def build_theme_uploads_hash
     hash = {}
-    upload_fields.each do |field|
-      hash[field.name] = Discourse.store.cdn_url(field.upload.url) if field.upload&.url
-    end
+    upload_fields
+      .includes(:javascript_cache, :upload)
+      .each do |field|
+        hash[field.name] = Discourse.store.cdn_url(field.upload.url) if field.upload&.url
+      end
     hash
   end
 
   def build_local_theme_uploads_hash
     hash = {}
-    upload_fields.each do |field|
-      hash[field.name] = field.javascript_cache.local_url if field.javascript_cache
-    end
+    upload_fields
+      .includes(:javascript_cache, :upload)
+      .each do |field|
+        hash[field.name] = field.javascript_cache.local_url if field.javascript_cache
+      end
     hash
   end
 
@@ -843,6 +847,8 @@ class Theme < ActiveRecord::Base
         if upload = field.upload
           url = upload_cdn_path(upload.url)
           contents << "$#{field.name}: unquote(\"#{url}\");"
+        else
+          contents << "$#{field.name}: unquote(\"\");"
         end
       else
         contents << to_scss_variable(field.name, field.value)