fix: correct wrong fix, get test to actually do something (#16779)

#16773 added a test with _expected.html to a wrong suit, so it was ignored, and this allowed to slip in a new bug of printing the falsy hidden attribute as hidden (shortened enabled form).
That fix wasn't released yet, so no changeset.

packages/svelte/src/internal/shared/attributes.js

@@ -22,11 +22,11 @@ const replacements = {
  * @returns {string}
  */
 export function attr(name, value, is_boolean = false) {
-	if (value == null || (!value && is_boolean)) return '';
 	// attribute hidden for values other than "until-found" behaves like a boolean attribute
 	if (name === 'hidden' && value !== 'until-found') {
 		is_boolean = true;
 	}
+	if (value == null || (!value && is_boolean)) return '';
 	const normalized = (name in replacements && replacements[name].get(value)) || value;
 	const assignment = is_boolean ? '' : `="${escape_html(normalized, true)}"`;
 	return ` ${name}${assignment}`;