remove invalid exception when no plugins are found

E.g. when running the nodelet manager but no nodelets are there it raises an exception.

It  looks like this regression was introduces in this refactoring: https://github.com/ros/pluginlib/commit/0ce6b4744101fb1379b07709a0f6b082e49c7873#diff-4a232a6b10422dba01b32b3bb76b933bR182

Before there was no exception then the result list was just empty.