GeocodeLib.php _transform() handles null results

I have run into this too, if the data doesn't come back from the geocode API for whatever reason, the `_transform()` doesn't work.

https://travis-ci.org/dereuromark/cakephp-tools/jobs/38745456
"array_key_exists() expects parameter 2 to be array, null given"

added a couple of minor checks to ensure that an array is what we're dealing with, for results, at all times