Remove PHP warning when an unknown host is requested

When using an unknown host in queries, for example when going to ``cgp/host.php?h=anUnknownHostWhichDoesNotExist``, PHP displays following warning right before a line containing ``Unknown host``:

    Warning: Invalid argument supplied for foreach() in /srv/http/cgp/inc/collectd.inc.php on line 93

This warning comes from ``collectd_plugins()`` which uses the return value of ``collectd_plugindata()`` in a ``foreach`` statement even though this value may be ``false``. My commit removes the warning by directly returning an empty array if this value is ``false`` or empty. Feel free to change "empty array" to something else (like ``false``?) if you think it's better.