Previously, (gexp-native-inputs #~#$@(list #~#+foo)) would return '().
This is a followup to 5b14a7902c58d9fb7923f9e16871f549fbe59b6e.
* guix/gexp.scm (gexp-inputs)[add-reference-inputs]: In the list case,
remove 'if' around 'fold-right'. In 'map' lambda, always inherit N?.
* tests/gexp.scm ("gexp list splicing + ungexp-splicing"): New test.
