Form validation: require at least one attribute

What does this MR do and why?

Form validation: require at least one attribute

Require at least one attribute in order to save a category

References

• original issue, for which this is a follow-up change: Frontend: Hook up category attribute configurat... (#550582 - closed)
• epic: Security Attributes/Context Filtering (&18010)

Screenshots or screen recordings

Before	After
required fields: name, selection type	required fields: name, selection type, at least one attribute

How to set up and validate locally

git checkout mfluharty-require-at-least-one-attribute-to-save-a-category
echo "Feature.enable(:security_categories_and_attributes)" | gdk rails c
echo "Feature.enable(:security_context_labels)" | gdk rails c

1. navigate to a group where you are at least a maintainer => Secure => Security configuration
2. try to create a new blank category, you should be unable to save the category when it has 0 attributes

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