Ensure PROTOCOL_HANDLER system property is reset

See gh-31870

spring-boot-project/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/jar/Handler.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2022 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -413,7 +413,7 @@ public class Handler extends URLStreamHandler {
 	 */
 	static void captureJarContextUrl() {
 		if (canResetCachedUrlHandlers()) {
-			String handlers = System.getProperty(PROTOCOL_HANDLER, "");
+			String handlers = System.getProperty(PROTOCOL_HANDLER);
 			try {
 				System.clearProperty(PROTOCOL_HANDLER);
 				try {