bug: nil as database/sql arg transforms to Void, but must be a NullType

[asmyasnikov]

result, err := db.ExecQuery(ctx, "INSERT INTO tbl (a, b, c) VALUES ($p1, $p2, $p3)", 
  sql.Named("p1", 1),
  sql.Named("p2", nil), // produced Void
  sql.Named("p3", 3),
)

YDB not accepts Void value for insert into Optional column
Special type NullType helps to accept nil from user and apply to YDB on INSERT queries