fix dictionary.__repr__ for no user-dict case

[mh-northlander]

current __repr__ misses close "]" when the user dict list is empty:

dict = sudachipy.Dictionary()
dict.__repr__()
# '<SudachiDictionary(system=/path/to/system.dic, user=[)>'

[mh-northlander]

Vec<T> does not have the display trait and we need to write.
https://doc.rust-jp.rs/rust-by-example-ja/hello/print/print_display.html

I chose to branch at index=0.