Make cider-repl-set-ns to ask for ns in buffers with no ns form

Author: vspinu

cider-repl.el

@@ -659,7 +659,8 @@ text property `cider-old-input'."
 
 If invoked in a REPL buffer the command will prompt you for the name of the
 namespace to switch to."
-  (interactive (list (if (derived-mode-p 'cider-repl-mode)
+  (interactive (list (if (or (derived-mode-p 'cider-repl-mode)
+                             (null (cider-ns-form)))
                          (completing-read "Switch to namespace: "
                                           (cider-sync-request:ns-list))
                        (cider-current-ns))))