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

```
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

adapter/sqlite3/adapter.go

@@ -110,9 +110,18 @@ func (a *Adapter) getForeignKeys(tableName string) ([]*db.ForeignKey, error) {
 			return nil, err
 		}
 
+		toColumn := ""
+		if row["to"] == nil {
+			// NOTE: If `to` is NULL, implicitly equals `id` (maybe...)
+			// c.f. https://github.com/diesel-rs/diesel/issues/1535
+			toColumn = "id"
+		} else {
+			toColumn = row["to"].(string)
+		}
+
 		foreignKey := &db.ForeignKey{
 			FromColumn: row["from"].(string),
-			ToColumn:   row["to"].(string),
+			ToColumn:   toColumn,
 			ToTable:    row["table"].(string),
 		}
 

adapter/sqlite3/adapter_test.go

@@ -73,6 +73,19 @@ func TestAdapter_GetTable(t *testing.T) {
 		a.DB.MustExec("CREATE UNIQUE INDEX index_user_id_and_target_user_id_on_followers ON followers(user_id, target_user_id)")
 		a.DB.MustExec("CREATE UNIQUE INDEX index_target_user_id_and_user_id_on_followers ON followers(target_user_id, user_id)")
 
+		a.DB.MustExec(`
+			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)
+			);`)
+
 		type args struct {
 			tableName string
 		}
@@ -189,6 +202,46 @@ func TestAdapter_GetTable(t *testing.T) {
 					},
 				},
 			},
+			{
+				name: "album_genres",
+				args: args{
+					tableName: "album_genres",
+				},
+				want: &db.Table{
+					Name: "album_genres",
+					Columns: []*db.Column{
+						{
+							Name:    "album_id",
+							Type:    "varchar",
+							NotNull: true,
+						},
+						{
+							Name:    "genre_id",
+							Type:    "varchar",
+							NotNull: true,
+						},
+					},
+					ForeignKeys: []*db.ForeignKey{
+						{
+							FromColumn: "genre_id",
+							ToTable:    "genre",
+							ToColumn:   "id",
+						},
+						{
+							FromColumn: "album_id",
+							ToTable:    "album",
+							ToColumn:   "id",
+						},
+					},
+					Indexes: []*db.Index{
+						{
+							Name:    "sqlite_autoindex_album_genres_1",
+							Columns: []string{"album_id", "genre_id"},
+							Unique:  true,
+						},
+					},
+				},
+			},
 		}
 
 		for _, tt := range tests {