Fix for issue 2910 (uninstall : javaexception when specifying bad admin dn)

The fix consists of checking whether the uid and pwd arguments are null or not.

Appart from this a bug has been fixed in the uninstall parser: the method parseArguments was called twice (once in UninstallLauncher and another in UninstallCliHelper).  This made that providing any non-boolean argument in the command-line resulted in a error.