Fix /api/v1/conversations sometimes returning empty accounts (#25499)

app/models/account_conversation.rb

@@ -32,14 +32,8 @@ class AccountConversation < ApplicationRecord
   end
 
   def participant_accounts
-    @participant_accounts ||= begin
-      if participant_account_ids.empty?
-        [account]
-      else
-        participants = Account.where(id: participant_account_ids).to_a
-        participants.empty? ? [account] : participants
-      end
-    end
+    @participant_accounts ||= Account.where(id: participant_account_ids).to_a
+    @participant_accounts.presence || [account]
   end
 
   class << self

spec/controllers/api/v1/conversations_controller_spec.rb

@@ -18,6 +18,7 @@ RSpec.describe Api::V1::ConversationsController do
 
     before do
       PostStatusService.new.call(other.account, text: 'Hey @alice', visibility: 'direct')
+      PostStatusService.new.call(user.account, text: 'Hey, nobody here', visibility: 'direct')
     end
 
     it 'returns http success' do
@@ -33,7 +34,8 @@ RSpec.describe Api::V1::ConversationsController do
     it 'returns conversations' do
       get :index
       json = body_as_json
-      expect(json.size).to eq 1
+      expect(json.size).to eq 2
+      expect(json[0][:accounts].size).to eq 1
     end
 
     context 'with since_id' do
@@ -41,7 +43,7 @@ RSpec.describe Api::V1::ConversationsController do
         it 'returns conversations' do
           get :index, params: { since_id: Mastodon::Snowflake.id_at(1.hour.ago, with_random: false) }
           json = body_as_json
-          expect(json.size).to eq 1
+          expect(json.size).to eq 2
         end
       end