Fixed error for backend menu "H&O Import"

If no import is defined in the XML and you call the backend page "System > Import/Export > H&O Import", you get an error:

Fatal error: Call to a member function asArray() on a non-object in /path/to/magento/app/code/local/Ho/Import/Model/Resource/System/Import/Collection.php on line 80

This fix checks whether a node is returned.
Merge pull request #8 from mzeis/patch-2

Fixed error for backend menu "H&O Import"