writer: provide option where null values are just empty.

[Giszmo]

If empty gets read as null, it should at least optionally be possible to also write null as empty.

$ dump(load("something: "))
something: null

[puzrin]

https://github.com/nodeca/js-yaml#safedump-object---options-

There are styles to select dumping formats when possible. IMO you can create schema with customize null type, if existing not enough

[Giszmo]

I think I could help myself but it looks hacky:

const schema = yaml.DEFAULT_SAFE_SCHEMA
schema.compiledTypeMap.scalar['tag:yaml.org,2002:null'].represent.lowercase = function() { return '' }
file.write(`---
${yaml.safeDump(header, {
  schema: schema
})}
...

[rlidwka]

Fixed in 421ed22 (located in dev branch at the moment).

Normally, I'd advise to override null schema. But some other parsers (namely, psych and ruamel.yaml) serialize null this way, so it makes sense to at least provide an option.