FIX: remove nil items before sorting the sha1 string array. (#18571)

Previously, when the array had both nil and string values it returned the error "comparison of NilClass with String failed". Now I added the `.compact` method to prevent this issue as per @martin-brennan's suggestion https://github.com/discourse/discourse/pull/18431#discussion_r984204788

lib/stylesheet/manager/builder.rb

@@ -225,7 +225,7 @@ class Stylesheet::Manager::Builder
       sha1s << upload_field.upload&.sha1
     end
 
-    Digest::SHA1.hexdigest(sha1s.sort!.join("\n"))
+    Digest::SHA1.hexdigest(sha1s.compact.sort!.join("\n"))
   end
 
   def default_digest