Fix the check on select2 translations asset existence

In environment expecting precompiled assets `Rails.application.assets`
is `nil` and we should fall back on checking the manifest.

(cherry picked from commit 8f997be)

backend/app/views/spree/admin/shared/_js_locale_data.html.erb

@@ -33,7 +33,10 @@
 <script data-hook='admin-custom-translations'>
 </script>
 
-<% if I18n.locale != :en && I18n.locale %>
-  <% select2_locale_path = "solidus_admin/select2_locales/select2_locale_#{I18n.locale}" %>
-  <%= javascript_include_tag select2_locale_path, data: {turbolinks_track: 'reload'} if Rails.application.assets.find_asset(select2_locale_path) %>
+<% select2_locale_path = "solidus_admin/select2_locales/select2_locale_#{I18n.locale}.js" %>
+<% if I18n.locale != :en && I18n.locale && (
+    Rails.application.assets&.find_asset(select2_locale_path) || # compiled on the fly
+    Rails.application.assets_manifest.assets[select2_locale_path] # precompiled
+) %>
+  <%= javascript_include_tag select2_locale_path, data: {turbolinks_track: 'reload'} %>
 <% end %>