Quick fix for init.d script to stop error at stop

Wrapped kill call in a check for empty.

Couldn't get -z $PLPLUG to work - had to fall back to X"" != X"$PLPLUG" - perhaps readynas bash is too old?

The plugins are usually killed by the stopping of the server - so perhaps the kill line is unneeded - but I left it in just in case something gets left behind - it will only run if it finds something to kill.