Fix backdrop rootElement not initialized in Modal (#33853)

* Initialize default value of rootElement before using

* Remove redundant test | put rootElement tests together

Co-authored-by: GeoSot <geo.sotis@gmail.com>

js/src/util/backdrop.js

@@ -89,6 +89,8 @@ class Backdrop {
       ...Default,
       ...(typeof config === 'object' ? config : {})
     }
+
+    config.rootElement = config.rootElement || document.body
     typeCheckConfig(NAME, config, DefaultType)
     return config
   }