services: pipefs: Correctly handle lack of extensions.

Previously, in the absence of extensions of 'pipefs-service-type', we'd
get a wrong-type-arg exception while folding services:

  In gnu/services/nfs.scm:
     134:37  1 (_ #<<pipefs-configuration> mount-point: "/var/lib/nfs…> …)
  In ice-9/boot-9.scm:
    1685:16  0 (raise-exception _ #:continuable? _)
  In procedure car: Wrong type argument in position 1 (expecting pair): ()

* gnu/services/nfs.scm (pipefs-service-type)[extend]: Gracefully handle
the case where VALUES is the empty list, as done in 'gss-service-type'.

0 files changed, 0 insertions, 0 deletions