scripts: Commands warn when passed zero arguments.

This is a followup to 3f8326237d.

* guix/scripts/archive.scm (export-from-store): Warn then FILES is
empty.
* guix/scripts/build.scm (guix-build): Likewise.
* guix/scripts/copy.scm (warn-if-empty): New procedure.
(send-to-remote-host, retrieve-from-remote-host): Call it.
* guix/scripts/edit.scm (guix-edit): Warn when SPECS is empty.
* guix/scripts/environment.scm (guix-environment): Warn when MANIFEST
has zero entries.
* guix/scripts/graph.scm (guix-graph): Warn then ITEMS is empty.
* guix/scripts/package.scm (process-actions): Warn when FILES and TRANS
are both empty.

guix/scripts/build.scm

@@ -679,6 +679,9 @@ needed."
                                         (_ #f))
                                       opts)))
 
+              (when (null? items)
+                (warning (G_ "no arguments specified, nothing to build~%")))
+
               (cond ((assoc-ref opts 'log-file?)
                      ;; Pass 'show-build-log' the output file names, not the
                      ;; derivation file names, because there can be several