doors: fix "lb set tags" command with no arguments

Motivation:

Commit 9a83eb3 introduced the possibility to run the "lb set tags"
command without any arguments.  This should clear all tags for this
door.

Unfortunately that patch was buggy, as the default value was null,
resulting in a NullPointerException.

Modification:

Introduce an empty list as the default value.

Result:

A bug is fixed where running the "lb set tags" admin command without any
arguments triggers a NullPointerException.

Target: master
Request: 6.0
Request: 5.2
Request: 5.1
Request: 5.0
Request: 4.2
Requires-notes: yes
Requires-book: no
Patch: https://rb.dcache.org/r/12075/
Acked-by: Tigran Mkrtchyan
Acked-by: Lea Morschel

modules/cells/src/main/java/dmg/cells/services/login/LoginBrokerPublisher.java

@@ -140,7 +140,7 @@ public String call()
     class SetTagCommand implements Callable<String>
     {
         @Argument(required = false)
-        String[] tags;
+        String[] tags = {};
 
         @Override
         public String call()