implement Terraform null semantics to avoid complex code

[gotwarlost]

This PR implements Terraform semantics for null attribute values, which is to remove them from the output.

Currently it is difficult to create optional attributes and must be done like so:

    spec = merge( { actual: spec}, optionalAttributeNeeded ? { foo: "bar"} : {}

All this is unnecessary given that k8s objects do not have explicit nulls in their spec. So filtering out null values makes the above code look like:

   spec = { actual: spec, foo: optionalAttributeNeeded ? "bar" : null }

[gotwarlost]

superseded by #74