encoding/yaml: add options to omit empty or null documents in streams

[mvdan]

As #3464 points out, it is possible for YAML files to include empty or null documents:

exec cue import --outfile=- --list input.yaml

-- input.yaml --
---
kind: Deployment
---
---
kind: Deployment
---
kind: Other
---
null

Currently, both are mapped to a null value:

> exec cue import --outfile=- --list input.yaml
[stdout]
[{
	kind: "Deployment"
}, null, {
	kind: "Deployment"
}, {
	kind: "Other"
}, null]

This is good as a default, as it doesn't lose any information, and one can filter out those null top-level values if they wish to. However, when dealing with other tools which produce YAML like this, it can be a pain point to have to do this filtering as an extra step.

We propose adding options to the YAML encoding: omitempty would skip over documents in a stream which are empty (or only comments), and omitnull would skip over documents in a stream which are empty or the null value, as they are both equally null.

Then, one could do, for example:

> exec cue import --outfile=- --list yaml+omitnull: input.yaml
[stdout]
[{
	kind: "Deployment"
}, {
	kind: "Deployment"
}, {
	kind: "Other"
}]

It is possible that such options could also be added to other streaming encodings, although right now I can't think of any. JSONL or NDJSON, for example, does technically map each line to one JSON value - but we currently skip over empty lines due to how Go's encoding/json decoder works, so it's as if omitempty were always set, as we don't fail on empty lines.

[mvdan]

Worth noting that this is not urgent given that we are resolving #3464 with a change to the cue import --path flag.