substitute: Honor client-provided empty URL list.

Before that, 'guix build --substitute-urls=""' would lead to using the
daemon's own URL list instead of the empty list.  The 'or*' hack, which
is to blame, had become unnecessary since commit
fb4bf72be3.

Reported by Mark H Weaver <mhw@netris.org>.

* guix/scripts/substitute.scm (or*): Remove.
(%cache-urls): Use 'or' instead of 'or*'.
* tests/store.scm ("substitute query, alternating URLs"): Add test with
empty URL list.
* doc/guix.texi (Common Build Options): Mention the empty string.

doc/guix.texi

@@ -3983,6 +3983,9 @@ This means that substitutes may be downloaded from @var{urls}, provided
 they are signed by a key authorized by the system administrator
 (@pxref{Substitutes}).
 
+When @var{urls} is the empty string, substitutes are effectively
+disabled.
+
 @item --no-substitutes
 Do not use substitutes for build products.  That is, always build things
 locally instead of allowing downloads of pre-built binaries

guix/scripts/substitute.scm

@@ -953,15 +953,9 @@ substitutes may be unavailable\n")))))
 found."
   (assoc-ref (daemon-options) option))
 
-(define-syntax-rule (or* a b)
-  (let ((first a))
-    (if (or (not first) (string-null? first))
-        b
-        first)))
-
 (define %cache-urls
-  (match (and=> (or* (find-daemon-option "untrusted-substitute-urls") ;client
-                     (find-daemon-option "substitute-urls"))          ;admin
+  (match (and=> (or (find-daemon-option "untrusted-substitute-urls") ;client
+                    (find-daemon-option "substitute-urls"))          ;admin
                 string-tokenize)
     ((urls ...)
      urls)

tests/store.scm

@@ -450,7 +450,11 @@
            (with-store s                        ;the right one again
              (set-build-options s #:use-substitutes? #t
                                 #:substitute-urls (%test-substitute-urls))
-             (has-substitutes? s o))))))
+             (has-substitutes? s o))
+           (with-store s                        ;empty list of URLs
+             (set-build-options s #:use-substitutes? #t
+                                #:substitute-urls '())
+             (not (has-substitutes? s o)))))))
 
 (test-assert "substitute"
   (with-store s