scripts: Commands warn when passed zero arguments.

scripts: Commands warn when passed zero arguments.

This is a followup to 3f8326237df780404c172ef4127195cc20becd66.

* 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.