The regression was introduced with a2b89a3319,
and introduce a risk that 'unset leaks to serialized configuration files.
* tests/services/configuration.scm (maybe-symbol): New type.
(config-with-maybe-symbol): New configuration.
("symbol maybe value serialization, unspecified"): New test.
Reported-by: Attila Lendvai <attila@lendvai.name>