Iterate correctly over multiple lists.

If any list in an implict iteration is null or contains only null-values,
then the promise shall be skipped entirely.

The previous logic tried to account for (and to some degree, rely
on cf_null) in the list of variable values (var_states). The resulting
logic was overly complicated and didn't correctly abort iteration if
any of the lists was null.

Instead, store in the iterator if we encounter a list that has no
valid values, and implement NullIterators (a function declared
in iterate.h but never implemented) to return true if that was the case
so that we can skip the entire promise.

Remove dead code. This goes a long way towards a complete purging of
cf_null. We can't do so completely as we need to account for policies
that explicitly insert cf_null (many tests), and for functions. If we
were to remove it, then rval.item being NULL would become a valid case,
but a lot of evaluation code treats that as an error.

Fixes Redmine #6553
Refactoring: Remove cf_null handling from functions.

Instead of making sure in every list-returning function that we don't
return the null-pointer, centralize the appending of CF_NULL_VALUE
in FnCallEvaluate.

Remove unnecessary null-pointer checks. Allow sum() and product() to
successfully calculate 0 and 1 for null-lists, respectively.