FIX: Do not error when importing a theme with empty files

app/models/remote_theme.rb

@@ -149,7 +149,7 @@ class RemoteTheme < ActiveRecord::Base
     end
 
     # Destroy fields that no longer exist in the remote theme
-    field_ids_to_destroy = theme.theme_fields.pluck(:id) - updated_fields.map(&:id)
+    field_ids_to_destroy = theme.theme_fields.pluck(:id) - updated_fields.map { |tf| tf&.id }
     ThemeField.where(id: field_ids_to_destroy).destroy_all
 
     if !skip_update

spec/models/remote_theme_spec.rb

@@ -47,6 +47,7 @@ describe RemoteTheme do
         "about.json" => about_json,
         "desktop/desktop.scss" => scss_data,
         "scss/file.scss" => ".class1{color:red}",
+        "scss/empty.scss" => "",
         "common/header.html" => "I AM HEADER",
         "common/random.html" => "I AM SILLY",
         "common/embedded.scss" => "EMBED",