Skip over empty (cf_null) list entries in maplist.

Fixes Redmine #6576, includes acceptance test.
Or not. The right-hand side of `expression` in a classes promise doesn't actually support this kind of regex, so the only way around this would be to use `classesmatching`, which introduces circular dependencies between variables and classes, so a multi-pass situation that overcomplicates the test case. Merging as is.