Fix: LD Signaturesで署名された投稿の検索許可（検索範囲）が改竄できる問題

spec/services/activitypub/process_account_service_spec.rb

@@ -49,7 +49,7 @@ RSpec.describe ActivityPub::ProcessAccountService, type: :service do
     end
 
     context 'when direct' do
-      let(:searchable_by) { '' }
+      let(:searchable_by) { 'https://foo.test' }
 
       it 'searchability is direct' do
         expect(subject.searchability).to eq 'direct'
@@ -72,6 +72,14 @@ RSpec.describe ActivityPub::ProcessAccountService, type: :service do
       end
     end
 
+    context 'when empty array' do
+      let(:searchable_by) { '' }
+
+      it 'searchability is direct' do
+        expect(subject.searchability).to eq 'direct'
+      end
+    end
+
     context 'when default value' do
       let(:searchable_by) { nil }