Fix crash when conversations have no valid participants (#10078)

* Never return empty participants for conversations

Fixes #10068

* Fix client-side crash when conversations have no participants

app/javascript/mastodon/components/status.js

@@ -326,7 +326,7 @@ class Status extends ImmutablePureComponent {
       );
     }
 
-    if (otherAccounts) {
+    if (otherAccounts && otherAccounts.size > 0) {
       statusAvatar = <AvatarComposite accounts={otherAccounts} size={48} />;
     } else if (account === undefined || account === null) {
       statusAvatar = <Avatar account={status.get('account')} size={48} />;