Allow the plugin libraries to have empty path for location

When specifying a path for the plugin location folder, a slash used to be appended to the path.

According to dlopen manual:

    If  filename  contains  a  slash ("/"), then it is interpreted as a (relative or absolute) pathname


So when we were specifying an empty location, we wanted to use the default location. Due to the slash appending, `dlopen()` was searching for plugin names in `/`, which obviously fails.