[Sqlite3] Fixed. panic: interface conversion when foreign key to column is nil

[sue445]

Schema causing the error

ref. #210

sqlite> .schema album_genres
CREATE TABLE album_genres
(
	album_id varchar default null not null
		references album
			on delete cascade,
	genre_id varchar default null not null
		references genre
			on delete cascade,
	constraint album_genre_ux
		unique (album_id, genre_id)
);

sqlite> PRAGMA foreign_key_list(album_genres) ;
       id = 0
      seq = 0
    table = genre
     from = genre_id
       to =
on_update = NO ACTION
on_delete = CASCADE
    match = NONE

       id = 1
      seq = 0
    table = album
     from = album_id
       to =
on_update = NO ACTION
on_delete = CASCADE
    match = NONE

Close #210