Replace deprecated NullOutputStream (#4989)

Co-authored-by: Raihaan Shouhell <raihaanhimself@gmail.com>

core/src/main/java/hudson/ClassicPluginStrategy.java

@@ -535,7 +535,7 @@ public class ClassicPluginStrategy implements PluginStrategy {
 
         final long dirTime = archive.lastModified();
         // this ZipOutputStream is reused and not created for each directory
-        try (ZipOutputStream wrappedZOut = new ZipOutputStream(new NullOutputStream()) {
+        try (ZipOutputStream wrappedZOut = new ZipOutputStream(NullOutputStream.NULL_OUTPUT_STREAM) {
             @Override
             public void putNextEntry(ZipEntry ze) throws IOException {
                 ze.setTime(dirTime+1999);   // roundup