fix random rosstack bug which was causing an opendir error if ROS_PACKAGE_PATH was not defined; this was because my string_split function was returning a 1-element vector (with an empty string) if it was passed an empty string.
fix random rosstack bug which was causing an opendir error if ROS_PACKAGE_PATH was not defined; this was because my string_split function was returning a 1-element vector (with an empty string) if it was passed an empty string.

ros/ros,ros/ros,ros/ros