fix: return null if parseYaml does not yield any result

[netomi]

This fixes #714 .

The documentation is not clear what shall happen when parsing a yaml string does not result in anything.
The fix now uses null as return value, which would be consistent with jrsonnet for this example.

An empty object would also be reasonable imho.

btw. the same problem exists for parseJson.

[coveralls]

coverage: 44.348% (+0.01%) from 44.335%
when pulling ad1fccb on netomi:fix-parse-yaml
into fb1ca81 on google:master.

[johnbartholomew]

As the original bug report notes, it's not really clear what the output should be. I can see arguments for producing an error (but not a Go panic/internal error!), returning null, maybe even other things.

It looks like std.parseJson produces an error if given empty input:

$ go run ./cmd/jsonnet -e 'std.parseJson("")'
RUNTIME ERROR: failed to parse JSON: unexpected end of JSON input
	<cmdline>:1:1-18	$
	During evaluation

Anyway, I guess null is ok, so I'll go ahead with this.

[He-Pin]

in sjsonnet, it's {}, but I think null is right here.