Inconsistency of DATETIME between TiDB and MySQL

## Bug Report

### 1. Minimal reproduce step (Required)

```sql
SELECT CAST('2020-01-01 a' AS DATETIME);
-- MySQL: 2020-01-01 00:00:00
-- TiDB: Null
```

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

TiDB keeps the same behavior with MySQL since mentioned in the [Doc](https://docs.pingcap.com/tidb/stable/cast-functions-and-operators/#cast).

Or Document should emphasize this inconsistency.

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

```sql
-- MySQL: 2020-01-01 00:00:00
-- TiDB: Null
```

### 4. What is your TiDB version? (Required)

```
tiup playground nightly --host 0.0.0.0 --kv 3 --tiflash 3
```
```
8.0.11-TiDB-v9.0.0-beta.2.pre-384-g5fab436
```