crash parsing explicit keys/empty values

edit:

```yaml
{
? explicit key1 : explicit value,
? explicit key2 : , # Explicit empty
? explicit key3,     # Empty value
simple key1 : explicit value,
simple key2 : ,     # Explicit empty
simple key3,         # Empty value
}
```
see https://github.com/KOLANICH/YAML.fts/blob/master/ruamel.yaml/canonical/spec-10-10.yaml