core: fix NPE in TryCatchTemplate

Motivation:

The TryCatchTemplate is meant to be composable, allowing different
components to "wrap" an existing TryCatchTemplate.  However, this
currently only works if the instantiated TryCatchTemplate overrides the
`execute` or `executeWithCancellable` methods.  "Out of the box" the
class results in a NPE.

Modification:

Update TryCatchTemplate so it doesn't throw NPE when neither the
`execute` or `executeWithCancellable` method is overridden.

Result:

No user or admin observable changes, but the code behaves more
correctly.

Target: master
Requires-notes: no
Requires-book: no
Patch: https://rb.dcache.org/r/13097/
Acked-by: Albert Rossi
Acked-by: Lea Morschel

modules/dcache/src/main/java/org/dcache/util/TryCatchTemplate.java

@@ -19,6 +19,8 @@
 
 import com.google.common.io.Closer;
 
+import javax.annotation.Nonnull;
+
 import java.io.Closeable;
 import java.nio.channels.CompletionHandler;
 
@@ -174,9 +176,10 @@ protected void execute() throws Exception
      * An implementation should not call {@code completed} or {@code setCancellable} from within
      * {@code executeWithCancellable}.
      */
+    @Nonnull
     protected Cancellable executeWithCancellable() throws Exception
     {
-        return null;
+        return msg -> {};
     }
 
     /**