Error: Flow sequence contains an unexpected ,

**version:**
1.0.0-beta.6

**input:**

```yaml
[ , ]
---
[ 123,, ]
```

**output:**

```
YAMLSyntaxError: Flow sequence contains an unexpected ,
YAMLSyntaxError: Flow sequence contains an unexpected ,
```

**expected:**

```json
[ null ]
---
[ 123, null ]
```

P.S. There's no error for its flowMap version but its `YAMLMap#items` is also empty.