fix: use window name to detect presence of extended client details (#22760)

RouteScope logic behaves differently depending on whether extended client details have been fetched.
Previously, the code checked whether the instance was null, but since #22719,
UIInternal.getExtendedClientDetails never returns null.

This change updates the logic to instead check ExtendedClientDetails.windowName,
which is always sent by the Flow client once the details are available.

Author: mcollovati

vaadin-spring/src/main/java/com/vaadin/flow/spring/scopes/VaadinRouteScope.java

@@ -78,7 +78,7 @@ private RouteBeanStore getBeanStore(UI ui) {
             ExtendedClientDetails details = ui.getInternals()
                     .getExtendedClientDetails();
             String key = getUIStoreKey(ui);
-            if (details == null) {
+            if (details.getWindowName() == null) {
                 ui.getPage().retrieveExtendedClientDetails(
                         det -> relocateStore(ui, key));
             }
@@ -119,10 +119,10 @@ private void relocateStore(UI ui, String key) {
         private String getUIStoreKey(UI ui) {
             ExtendedClientDetails details = ui.getInternals()
                     .getExtendedClientDetails();
-            if (details == null) {
+            if (details.getWindowName() == null) {
                 return "uid-" + ui.getUIId();
             } else {
-                return "win-" + getWindowName(ui);
+                return "win-" + details.getWindowName();
             }
         }
 
@@ -448,9 +448,6 @@ private static RouteBeanStore getBeanStoreIfExists(VaadinSession session) {
     private static String getWindowName(UI ui) {
         ExtendedClientDetails details = ui.getInternals()
                 .getExtendedClientDetails();
-        if (details == null) {
-            return null;
-        }
         return details.getWindowName();
     }