Use builtin.null_list in both list and dict is_empty

[Micrograx]

Small optimization for list and dict is_empty, after noticing this behavior in tests:

const l = []

// [mem:   1.13 K, cpu: 293.09 K]
test bench_is_empty() {
  list.is_empty(l)
}

// [mem:    532.0, cpu: 138.53 K]
test bench_builtin() {
  builtin.null_list(l)
}