Fix the error message if you pipe some data (e.g. echo | mco rpc ...) to be useful
stdin discovery plugin no longer needs fallback

We now only invoke the STDIN discovery plugin in the rpc application
when we're _sure_ there is data on STDIN for it.

Thus we can safely break if there is data which is only
whitespace in either case - we don't need a weird 'return zero hosts if whitespace only'
case for JSON any more, so I've removed it.