Ignore undefined props

[RemiBardon]

Do not render templates referencing undefined props. See stepci/stepci#220 for a use case explanation.

[mishushakov]

This is a weird behaviour. Liquid, which was the inspiration for this project just returns empty string:

https://liquidjs.com/playground.html#PHA+SGVsbG8sIHt7dGVzdH19IDwvcD4K,e30=

[RemiBardon]

I understand but at the same time there is no way to achieve the opposite behavior (not rendering a template). To get an empty string of the variable is not defined I'd expect the use of a filter like | default or something like that.

[RemiBardon]

Another solution for the $ref use case could be to render all children of $ref keys. That wouldn't require changing how liquidless works. I just thought about this, tell me if you'd prefer this solution.

[RemiBardon]

I just realized that while the solution I had proposed (and implemented) works, it does not work with recursive $refs. I had made an optimization on a project where a openapi.json file contained { "$ref": "http://${{ env.host }}/api-docs/openapi.json" } so the file would be resolved only once when running the tests, but it doesn't work in this scenario. I don't know how we could fix this though.