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'.

gnu/services/nfs.scm

@@ -131,7 +131,10 @@ use this facility."))))
      ;; configure and start this service.  Only one value can be provided.  We
      ;; override it with the value returned by the extending service.
      (compose identity)
-     (extend (lambda (config values) (first values)))
+     (extend (lambda (config values)
+               (match values
+                 ((first . rest) first)
+                 (_ config))))
      (default-value (pipefs-configuration))
      (description "Mount the pipefs file system, which is used to transfer
 NFS-related data between the kernel and user-space programs."))))