Fix admin recovery crash when blueprints.fields is null

[onlsol]

Summary

Admin can go into recovery mode with:

foreach() argument must be of type array|object, null given

when rendering blueprints where blueprints.fields is null.

Steps to reproduce

1. Use a blueprint that does not define any form/fields, so blueprints.fields ends up null.
2. Open the corresponding page in the Admin panel.
3. Admin switches to recovery mode with an exception pointing at partials/blueprints.html.twig line 7.

Technical details

In the Admin plugin there are two twig templates that call Admin::findFormFields() like this:

{% if admin.findFormFields('file', blueprints.fields) %}