Undefined variables not throwing an error

[wolfv]

I think we might want to rethink how we handle undefined variables.

For example, this is not throwing the appropriate Jinja error:

host:
  - ${{ asdasda }}

Instead, it is silently discarded.

[wolfv]

I looked into it and the issue is that minijinja supports only a mode where undefined values always error when we also want them to error when printing. For this reason, we should also set all non-matching platforms and arch's to false instead of relying on Undefined as falsey value.

I have openend a PR on rattler to be able to iterate through all possible values for Platform and Arch: conda/rattler#972