Add support for null checks

[juanjoDiaz]

YAML supports null (https://yaml.org/type/null.html).

It should be possible to do null checks like {{ if eq .myVar null }}.

[tazjin]

The code inside of template pipelines will refer to Go types and values, so you probably want == nil (where nil is what Go calls null).

Please note that the source of kontemplate is moving to git.tazj.in and issues should now be reported to the mailing list. Feel free to email depot@tazj.in if you need to follow up on this!