We'd like to disallow brackets and braces in our YAML, but there's a
catch: the only way to describe an empty array or hash in YAML is to
supply an empty one (`[]` or `{}`). Otherwise, the value will be null.
This commit adds a `non-empty` option to `forbid` for brackets and
braces. When it is set, all flow and sequence mappings will cause errors
_except_ for empty ones.
c897bf76a1