Add missing check to avoid re-initialization

Noticed during review of #28736 that a check protecting against
re-initialization was accidentally removed in commit
3d6e38b.

Author: rstoyanchev

spring-websocket/src/main/java/org/springframework/web/socket/config/annotation/WebSocketConfigurationSupport.java

@@ -84,12 +84,14 @@ protected void registerWebSocketHandlers(WebSocketHandlerRegistry registry) {
 	@Bean
 	@Nullable
 	public TaskScheduler defaultSockJsTaskScheduler() {
-		if (initHandlerRegistry().requiresTaskScheduler()) {
-			ThreadPoolTaskScheduler threadPoolScheduler = new ThreadPoolTaskScheduler();
-			threadPoolScheduler.setThreadNamePrefix("SockJS-");
-			threadPoolScheduler.setPoolSize(Runtime.getRuntime().availableProcessors());
-			threadPoolScheduler.setRemoveOnCancelPolicy(true);
-			this.scheduler = threadPoolScheduler;
+		if (this.scheduler == null) {
+			if (initHandlerRegistry().requiresTaskScheduler()) {
+				ThreadPoolTaskScheduler threadPoolScheduler = new ThreadPoolTaskScheduler();
+				threadPoolScheduler.setThreadNamePrefix("SockJS-");
+				threadPoolScheduler.setPoolSize(Runtime.getRuntime().availableProcessors());
+				threadPoolScheduler.setRemoveOnCancelPolicy(true);
+				this.scheduler = threadPoolScheduler;
+			}
 		}
 		return this.scheduler;
 	}