Handling of null, ~ and empty values

```yaml
- yes
- true
- false
- null
-
- ~
```
The values should be this:
```
=VAL :yes
=VAL :true
=VAL :false
=VAL :null
=VAL :
=VAL :~
```
Instead of ryml's current behavior:
```
=VAL :yes
=VAL :true
=VAL :false
=VAL :
=VAL :
=VAL :
```
See https://play.yaml.io/main/parser?input=LSB5ZXMKLSB0cnVlCi0gZmFsc2UKLSBudWxsCi0KLSB-Cg==
Thanks @ingydotnet