Introduces 'base-transducer', a SRFI-171 based transducer that can be used as a
starting point for writing custom configuration record serializing procedures.
This also fixes the symbol maybe-value serialization test case.
* gnu/services/configuration.scm (empty-serializer?): New predicate.
(base-transducer, tfilter-maybe-value): New procedure.
(serialize-configuration): Adapt to use base-transducer.
* gnu/services/telephony.scm (jami-account->alist): Use transducers to skip
fields that are unserializable or whose field maybe-value is unset.
* tests/services/configuration.scm: Remove test-expect-fail.
Signed-off-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>
