nis: fix ypdomainname configuration in ypbind init script

ypdomainname could return "" or "(none)" if the domain name is not
configured.  The ypbind init script does not check for "(none)" at
start up, so it thinks that the domain name is already configured.
The end result is that the domain name is not set per /etc/defaultdomain
and NIS initialialization fails.

Signed-off-by: Hong H. Pham <hong.pham@windriver.com>