Guard against build being missing from the output response

guix-data-service/builds.scm

@@ -234,6 +234,8 @@ initial connection on which HTTP requests are sent."
      (if data
          (catch #t
            (lambda ()
+             (when (assoc-ref data "build")
+               ;; The build can be #f
                (let* ((derivation
                        (assoc-ref data "derivation"))
                       (build-id
@@ -253,7 +255,7 @@ initial connection on which HTTP requests are sent."
                  (if (verbose-output?)
                      (simple-format #t "found build for: ~A (~A)\n"
                                     output derivation)
-                   (display "-"))))
+                     (display "-")))))
            (lambda (key . args)
              (simple-format
               (current-error-port)