Don't throw NPE when serving webjar directories

Prior to this change, serving resources with ResourceHttpRequestHandler
could result in NPE when requesting an existing folder located in a JAR.

This commit swallows those exceptions, as it is not possible to foresee
those cases without reading the actual resource. This result in a HTTP
200 response with a zero Content-Length instead of a HTTP 500 internal
exception.

Issue: SPR-13620

spring-webmvc/src/main/java/org/springframework/web/servlet/resource/ResourceHttpRequestHandler.java

@@ -460,6 +460,9 @@ public class ResourceHttpRequestHandler extends WebContentGenerator
 			try {
 				StreamUtils.copy(in, response.getOutputStream());
 			}
+			catch (NullPointerException ex) {
+				// ignore, see SPR-13620
+			}
 			finally {
 				try {
 					in.close();