[Vondry]

Next step in splitting #3751 towards Vue 3.

Why

Twig passes false for a field without a pattern or error message. Vue 2 drops a non-boolean attribute bound to false, but Vue 3 renders it as "false". On Vue 3 every such field would get:

• pattern="false", so the field only accepts the literal value "false" and the record can no longer be saved;
• data-errormessage="false", so common.js would show "false" as the validation message.

Why this approach

• The migration guide's fix is to bind null or undefined to drop an attribute. Both work on Vue 2.7 too, so this can land now.
• Only false is mapped. Vue 2 drops exactly false, null and undefined, so every other value ('', 0, any string) renders as before.
• A prop default would not help: Twig always passes the prop, so the default never applies.
• One helper keeps the reason and the link in a single place. In <script setup> the components will import it directly.

What changed

• assets/js/app/editor/utils/attributes.ts: optionalAttribute() returns undefined for false and every other value as it is.
• The editor fields bind data-errormessage and pattern through it: 19 bindings in Text, Email, Number, Password, Textarea, Slug, Embed, File, Image, Date and Select. placeholder is unchanged, because Twig sends '' for it, never false.
• Tests: each field checks that the attributes render when set and are dropped when false. Date, Select, File and Image use the tests from Migrate to Vue 3 Composition API and TypeScript #3751, replacing File's it.todo.