A11Y: Don't output aria-labelledby when card is empty (#22247)

The user card is always present in the DOM. Therefore we only need to
add the `aria-labelledby` attribute when there is a user (and a title)
to point to.

app/assets/javascripts/discourse/app/components/card-container.hbs

@@ -9,7 +9,6 @@
   @composePrivateMessage={{route-action "composePrivateMessage"}}
   @createNewMessageViaParams={{route-action "createNewMessageViaParams"}}
   role="dialog"
-  aria-labelledby="discourse-user-card-title"
 />
 
 <GroupCardContents

app/assets/javascripts/discourse/app/components/user-card-contents.js

@@ -30,6 +30,7 @@ export default Component.extend(CardContentsBase, CanCheckEmails, CleansUp, {
     "usernameClass",
     "primaryGroup",
   ],
+  attributeBindings: ["labelledBy:aria-labelledby"],
   allowBackgrounds: setting("allow_profile_backgrounds"),
   showBadges: setting("enable_badges"),
 
@@ -46,6 +47,11 @@ export default Component.extend(CardContentsBase, CanCheckEmails, CleansUp, {
   showDelete: and("viewingAdmin", "showName", "user.canBeDeleted"),
   linkWebsite: not("user.isBasic"),
 
+  @discourseComputed("user")
+  labelledBy(user) {
+    return user ? "discourse-user-card-title" : null;
+  },
+
   @discourseComputed("user")
   hasLocaleOrWebsite(user) {
     return user.location || user.website_name || this.userTimezone;