cannot show the condition with shard index in the PointGet

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

```
create table test3(id int primary key clustered, a int, b int, unique key uk_expr((tidb_shard(a)),a));
explain format=brief select * from test3 where a=100;
```

### 2. What did you expect to see? (Required)

```
id estRows task access object operator info
Projection 1.00 root explain_shard_index.test3.id, explain_shard_index.test3.a, explain_shard_index.test3.b
└─Point_Get 1.00 root table:test3, index:uk_expr(tidb_shard(`a`), a) condition: [eq(tidb_shard(explain_shard_index.test3.a), 8) eq(explain_shard_index.test3.a, 100)]
```
### 3. What did you see instead (Required)

```
id estRows task access object operator info
Projection 1.00 root explain_shard_index.test3.id, explain_shard_index.test3.a, explain_shard_index.test3.b
└─Point_Get 1.00 root table:test3, index:uk_expr(tidb_shard(`a`), a)
```
### 4. What is your TiDB version? (Required)