ui: Gracefully deal with zero-output derivations.

* guix/ui.scm (show-what-to-build)[built-or-substitutable?]: New
  procedure.  Check whether OUT is #f.
  Use it.
* tests/ui.scm ("show-what-to-build, zero outputs"): New test.

guix/ui.scm

@@ -261,6 +261,14 @@ error."
 derivations listed in DRV.  Return #t if there's something to build, #f
 otherwise.  When USE-SUBSTITUTES?, check and report what is prerequisites are
 available for download."
+  (define (built-or-substitutable? drv)
+    (let ((out (derivation->output-path drv)))
+      ;; If DRV has zero outputs, OUT is #f.
+      (or (not out)
+          (or (valid-path? store out)
+              (and use-substitutes?
+                   (has-substitutes? store out))))))
+
   (let*-values (((build download)
                  (fold2 (lambda (drv build download)
                           (let-values (((b d)
@@ -275,14 +283,7 @@ available for download."
                 ((build)                          ; add the DRV themselves
                  (delete-duplicates
                   (append (map derivation-file-name
-                               (remove (lambda (drv)
-                                         (let ((out (derivation->output-path
-                                                     drv)))
-                                           (or (valid-path? store out)
-                                               (and use-substitutes?
-                                                    (has-substitutes? store
-                                                                      out)))))
-                                       drv))
+                               (remove built-or-substitutable? drv))
                           (map derivation-input-path build))))
                 ((download)                   ; add the references of DOWNLOAD
                  (if use-substitutes?

tests/ui.scm

@@ -19,6 +19,8 @@
 
 (define-module (test-ui)
   #:use-module (guix ui)
+  #:use-module (guix store)
+  #:use-module (guix derivations)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-19)
   #:use-module (srfi srfi-64))
@@ -189,6 +191,16 @@ interface, and powerful string processing.")
     (lambda args
       #t)))
 
+(test-equal "show-what-to-build, zero outputs"
+  ""
+  (with-store store
+    (let ((drv (derivation store "zero" "/bin/sh" '()
+                           #:outputs '())))
+      (with-error-to-string
+       (lambda ()
+         ;; This should print nothing.
+         (show-what-to-build store (list drv)))))))
+
 (test-end "ui")