cells: update CellMessage to work with initially empty messages

Motivation:

Currently dcap uploads do not work because PoolManagerStub#send
sends the request with a CellMessage created with a zero-argument
constructor, and such a CellMessage has no destination.

Modification:

Update zero-args CellMessage to have an empty CellPath for destination
rather than null.  Update CellPath insert methods so they work with an
empty CellPath.

Result:

dcap uploads work again.

Target: master
Requires-notes: no
Requires-book: no
Patch: https://rb.dcache.org/r/9592/
Acked-by: Gerd Behrmann

modules/cells/src/main/java/dmg/cells/nucleus/CellMessage.java

@@ -64,14 +64,9 @@ public CellMessage(CellPath path, Serializable msg)
     }
 
     public CellMessage(CellPath path)
-    {
-        this();
-        _destination = path;
-    }
-
-    public CellMessage()
     {
         _source = new CellPath();
+        _destination = path;
         _creationTime = System.currentTimeMillis();
         _receivedAt = _creationTime;
         _mode = ORIGINAL_MODE;
@@ -80,6 +75,11 @@ public CellMessage()
         _session = CDC.getSession();
     }
 
+    public CellMessage()
+    {
+        this(new CellPath());
+    }
+
   @Override
   public String toString(){
     StringBuilder sb = new StringBuilder() ;

modules/cells/src/main/java/dmg/cells/nucleus/CellPath.java

@@ -115,11 +115,17 @@ public synchronized CellPath clone()
      */
     public synchronized void insert(CellPath path)
     {
+        if (_position < 0) {
+            _position = 0;
+        }
         _list.addAll(_position, path._list);
     }
 
     public synchronized void insert(CellAddressCore address)
     {
+        if (_position < 0) {
+            _position = 0;
+        }
         _list.add(_position, address);
     }