depot_autopilot.run: error message on empty tar

When wrongly invoking the run script by specifying a skipped test
as its only TEST_PKGS argument, the run script fails due to a wrong
tar argument order. Let's better reflect this condition to the user
ahead of invoking tar.

repos/gems/run/depot_autopilot.run

@@ -85,6 +85,11 @@ proc autopilot_create_tar_from_depot_binaries { archive_path args } {
 
 	check_for_missing_depot_archives
 
+	if {[llength $content] == 0} {
+		puts stderr "Error: no content for depot.tar"
+		exit -1
+	}
+
 	eval "exec tar cf $archive_path -T /dev/null -C [depot_dir] [lsort -unique $content]"
 }