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/display_name.js

@@ -22,7 +22,7 @@ export default class DisplayName extends React.PureComponent {
         suffix = `+${others.size - 2}`;
       }
     } else {
-      if (others) {
+      if (others && others.size > 0) {
         account = others.first();
       } else {
         account = this.props.account;

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} />;

app/models/account_conversation.rb

@@ -30,7 +30,8 @@ class AccountConversation < ApplicationRecord
     if participant_account_ids.empty?
       [account]
     else
-      Account.where(id: participant_account_ids)
+      participants = Account.where(id: participant_account_ids)
+      participants.empty? ? [account] : participants
     end
   end