print wrong flen with the VarString

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

```
create table t30094(a varchar(10));
explain format = 'brief' select * from t30094 where cast(a as float) and cast(a as char);
```

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

```
TableReader 8000.00 root data:Selection
└─Selection 8000.00 cop[tikv] cast(planner__core__integration.t30094.a, float BINARY), cast(planner__core__integration.t30094.a, var_string)

```
### 3. What did you see instead (Required)

```
TableReader 8000.00 root data:Selection
└─Selection 8000.00 cop[tikv] cast(planner__core__integration.t30094.a, float BINARY), cast(planner__core__integration.t30094.a, var_string(5))
```
### 4. What is your TiDB version? (Required)