Fix IndexOutOfBoundsException exception from parseUrl with empty spec

Update jar `Handler` code so that the `parseUrl` method can accept an
empty `spec`. Prior to this commit, a `classLoader.getResource("")`
call would result in a `null` result. This breaks a number of things
including `ClassPathResource` and `PathMatchingResourcePatternResolver`.

Fixes gh-38524

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

@@ -89,7 +89,7 @@ public class Handler extends URLStreamHandler {
 
 	private String extractContextPath(URL url, String spec, int start) {
 		String contextPath = url.getPath();
-		if (spec.charAt(start) == '/') {
+		if (spec.regionMatches(false, start, "/", 0, 1)) {
 			int indexOfContextPathSeparator = indexOfSeparator(contextPath);
 			if (indexOfContextPathSeparator == -1) {
 				throw new IllegalStateException("malformed context url:%s: no !/".formatted(url));