tidb_decode_key cannot decode every key in information_schema.tikv_region_status

## Bug Report

Please answer these questions before submitting your issue. Thanks!

tidb_decode_key cannot decode a key with empty row data (like `tid_r`), which appear for tables with TiFlash for example, since tiflash replicas only want table row data, no index data etc.

### 1. Minimal reproduce step (Required)

Use something like `tiup playground nightly` to setup the environment, including tiflash.

```sql
create table key_decode_test (a int primary key, b varchar(255));
select table_name, tidb_decode_key(start_key), tidb_decode_key(end_key), start_key, end_key from information_schema.tikv_region_status where table_name = 'key_decode_test';
alter table key_decode_test set tiflash replica 1;
select table_name, tidb_decode_key(start_key), tidb_decode_key(end_key), start_key, end_key from information_schema.tikv_region_status where table_name = 'key_decode_test';
```

Which will show something like `7480000000000000FF505F720000000000FA` instead of something like `{"handle":"{}","table_id":80}` (Notice that the handle value may be a string and not an actual json object)

### 2. What did you expect to see? (Required)
```
tidb> select table_name, tidb_decode_key(start_key), tidb_decode_key(end_key), start_key, end_key from information_schema.tikv_region_status where table_name = 'key_decode_test';
+-----------------+--------------------------------+--------------------------+--------------------------------------+--------------------------------------+
| table_name | tidb_decode_key(start_key) | tidb_decode_key(end_key) | start_key | end_key |
+-----------------+--------------------------------+--------------------------+--------------------------------------+--------------------------------------+
| key_decode_test | {"handle":{}} | {"table_id":81} | 7480000000000000FF505F720000000000FA | 7480000000000000FF5100000000000000F8 |
+-----------------+--------------------------------+--------------------------+--------------------------------------+--------------------------------------+
1 row in set (0.00 sec)
```
### 3. What did you see instead (Required)
```
tidb> create table key_decode_test (a int primary key, b varchar(255));
Query OK, 0 rows affected (0.10 sec)

tidb> select table_name, tidb_decode_key(start_key), tidb_decode_key(end_key), start_key, end_key from information_schema.tikv_region_status where table_name = 'key_decode_test';
+-----------------+----------------------------+------------------------------+--------------------------------------+--------------------------------------+
| table_name | tidb_decode_key(start_key) | tidb_decode_key(end_key) | start_key | end_key |
+-----------------+----------------------------+------------------------------+--------------------------------------+--------------------------------------+
| key_decode_test | {"table_id":80} | {"table_id":281474976710651} | 7480000000000000FF5000000000000000F8 | 748000FFFFFFFFFFFFFB00000000000000F8 |
+-----------------+----------------------------+------------------------------+--------------------------------------+--------------------------------------+
1 row in set (0.01 sec)

tidb> alter table key_decode_test set tiflash replica 1;
Query OK, 0 rows affected (0.08 sec)

tidb> select table_name, tidb_decode_key(start_key), tidb_decode_key(end_key), start_key, end_key from information_schema.tikv_region_status where table_name = 'key_decode_test';
+-----------------+--------------------------------------+--------------------------+--------------------------------------+--------------------------------------+
| table_name | tidb_decode_key(start_key) | tidb_decode_key(end_key) | start_key | end_key |
+-----------------+--------------------------------------+--------------------------+--------------------------------------+--------------------------------------+
| key_decode_test | 7480000000000000FF505F720000000000FA | {"table_id":81} | 7480000000000000FF505F720000000000FA | 7480000000000000FF5100000000000000F8 |
+-----------------+--------------------------------------+--------------------------+--------------------------------------+--------------------------------------+
1 row in set, 1 warning (0.00 sec)

tidb> alter table key_decode_test add index (b);
Query OK, 0 rows affected (0.54 sec)

tidb> select table_name, tidb_decode_key(start_key), tidb_decode_key(end_key), start_key, end_key from information_schema.tikv_region_status where table_name = 'key_decode_test';
+-----------------+--------------------------------------+--------------------------------------+--------------------------------------+--------------------------------------+
| table_name | tidb_decode_key(start_key) | tidb_decode_key(end_key) | start_key | end_key |
+-----------------+--------------------------------------+--------------------------------------+--------------------------------------+--------------------------------------+
| key_decode_test | {"table_id":80} | 7480000000000000FF505F720000000000FA | 7480000000000000FF5000000000000000F8 | 7480000000000000FF505F720000000000FA |
| key_decode_test | 7480000000000000FF505F720000000000FA | {"table_id":81} | 7480000000000000FF505F720000000000FA | 7480000000000000FF5100000000000000F8 |
+-----------------+--------------------------------------+--------------------------------------+--------------------------------------+--------------------------------------+
2 rows in set, 2 warnings (0.00 sec)
```

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