add a possibility to specify empty dicts in ConfigParser

[FoSix]

This is a simplification for use with Ansible. Sometimes to parametrise a check it would be easier to specify something like:

a_check:
  a_config: '{{ some_config_param |default(omit) }}'

which would translate to

a_check:

if the some_config_param is not defined. Unfortunately this is not supported. It's either a string (a_check) or a dict with a value (a_check: a_value)