Use existing window.dash_clientside if already set

Co-authored-by: Alex Johnson <[email protected]>

Author: jowlo

tests/integration/clientside/assets/clientside.mjs

@@ -1,5 +1,6 @@
 import {display} from "./clientsideModule.mjs";
 
+window.dash_clientside = window.dash_clientside || {};
 window.dash_clientside.clientside_module = Object.assign({}, window.dash_clientside.clientside_module, {
 	display
 });