Make namespace generation clearer

Let’s make it clear that this is where we initialize our namespaces to avoid
proliferation of various initialization points now that we support CommonJS.

preload.js

@@ -124,7 +124,7 @@
 
   const { IdleDetector} = require('./js/modules/idle_detector');
 
-  window.Signal = window.Signal || {};
+  window.Signal = {};
   window.Signal.Backup = require('./js/modules/backup');
   window.Signal.Crypto = require('./js/modules/crypto');
   window.Signal.Logs = require('./js/modules/logs');
@@ -134,7 +134,7 @@
   window.Signal.Migrations.upgradeMessageSchema = upgradeMessageSchema;
   window.Signal.Migrations.V17 = require('./js/modules/migrations/17');
   window.Signal.OS = require('./js/modules/os');
-  window.Signal.Types = window.Signal.Types || {};
+  window.Signal.Types = {};
   window.Signal.Types.Attachment = Attachment;
   window.Signal.Types.Errors = require('./js/modules/types/errors');
   window.Signal.Types.Message = Message;