[fio] allow empty path part in pathjoin (#3260)

Empty strings should be ignored, rather than throw an error.

Passing only empty strings (or nothing) to `pathjoin` will return '.' which means the current directory

Every path part passed to `pathjoin` is now converted to a string

The `gsub('/+', '/')` call already does what the removed code does, so avoid the unnecessary work
Simply check if the result equals '/' before removing a trailing '/'.

The previous check did extra work for no gain.