Improve brand kit config schema
>>> [!note] Migrated issue
<!-- Drupal.org comment -->
<!-- Migrated from issue #3580052. -->
Reported by: [mglaman](https://www.drupal.org/user/2416470)
Related to !751
>>>
<h3 id="overview">Overview</h3>
<p>Config schema improvements from <span class="drupalorg-gitlab-issue-link drupalorg-gitlab-link-wrapper"><a href="https://git.drupalcode.org/project/canvas/-/work_items/3571536" class="drupalorg-gitlab-link">https://git.drupalcode.org/project/canvas/-/work_items/3571536</a></span> can be applied to brand kits per <a href="https://git.drupalcode.org/project/canvas/-/merge_requests/667#note_722429">https://git.drupalcode.org/project/canvas/-/merge_requests/667#note_722429</a>. It was done in that MR, but let's do it in its own issue.</p>
<h3 id="proposed-resolution">Proposed resolution</h3>
<p>Remove `nullable: true` for:</p>
<pre> orderby: value<br> # Should be omitted when there are no imports.<br> requiredKey: false<br> constraints:<br> # There must be >=1 import; otherwise the key should be omitted.<br> NotBlank: ~</pre><h3 id="ui-changes">User interface changes</h3>
issue