installer: Fix wifi menu crash with hidden SSIDs.

This fixes https://issues.guix.gnu.org/issue/35622.

* gnu/installer/connman.scm (<service>): Mention that name may be false.
* gnu/installer/newt/wifi.scm (wifi-services): Filter out wifi services
without name.

Co-authored by: Pierre Neidhardt <mail@ambrevar.xyz>

gnu/installer/connman.scm

@@ -108,7 +108,7 @@
 (define-record-type* <service>
   service make-service
   service?
-  (name            service-name) ; string
+  (name            service-name) ; string or #f
   (type            service-type) ; string
   (path            service-path) ; string
   (strength        service-strength) ; integer

gnu/installer/newt/wifi.scm

@@ -144,6 +144,7 @@ of <service-item> records present in LISTBOX."
   (let ((services (connman-services)))
     (filter (lambda (service)
               (and (string=? (service-type service) "wifi")
+                   (service-name service)
                    (not (string-null? (service-name service)))))
             services)))