fix: validate empty object/array in ast Get

[liuq19]

validate empty object or array when get_by_path

performance:

goarch: amd64
pkg: github.com/bytedance/sonic/ast
cpu: Intel(R) Xeon(R) Platinum 8260 CPU @ 2.40GHz
               │ /tmp/tmpfnkd86xm.source.txt │  /tmp/tmpmbt6i4rj.target.txt  │
               │           sec/op            │   sec/op     vs base          │
GetOne_Sonic-8                   3.122µ ± 3%   3.086µ ± 6%  ~ (p=0.971 n=10)

               │ /tmp/tmpfnkd86xm.source.txt │  /tmp/tmpmbt6i4rj.target.txt   │
               │             B/s             │     B/s       vs base          │
GetOne_Sonic-8                  3.885Gi ± 4%   3.933Gi ± 5%  ~ (p=0.971 n=10)

               │ /tmp/tmpfnkd86xm.source.txt │  /tmp/tmpmbt6i4rj.target.txt   │
               │            B/op             │    B/op     vs base            │
GetOne_Sonic-8                    24.00 ± 0%   24.00 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal

               │ /tmp/tmpfnkd86xm.source.txt │  /tmp/tmpmbt6i4rj.target.txt   │
               │          allocs/op          │ allocs/op   vs base            │
GetOne_Sonic-8                    1.000 ± 0%   1.000 ± 0%  ~ (p=1.000 n=10) ¹

[AsterDY]

可以加个多级测试用例，例如

{`{"a":{"b":123}}`, []interface{}{"a", "c"}, ErrNotExist}

[liuq19]

好的，已经加了