Handle case when BroadcastMessage.current is nil

Somehow Rails.cache.fetch occasionally returns `nil` values, which causes
this endpoint to crash.

Closes #35094

lib/api/internal.rb

@@ -101,7 +101,7 @@ module API
       end
       get "/broadcast_message" do
-        if message = BroadcastMessage.current.last
+        if message = BroadcastMessage.current&.last
           present message, with: Entities::BroadcastMessage
         else
           {}

spec/requests/api/internal_spec.rb

@@ -35,6 +35,17 @@ describe API::Internal do
         expect(json_response).to be_empty
       end
     end
+
+    context 'nil broadcast message' do
+      it 'returns nothing' do
+        allow(BroadcastMessage).to receive(:current).and_return(nil)
+
+        get api('/internal/broadcast_message'), secret_token: secret_token
+
+        expect(response).to have_http_status(200)
+        expect(json_response).to be_empty
+      end
+    end
   end
   describe 'GET /internal/broadcast_messages' do