lang: core: embedded: provisioner: Fix panic

The conditions changed for this one, make it stricter.

lang/core/embedded/provisioner/main.mcl

@@ -558,7 +558,7 @@ class base:host($name, $config) {
 
 	$handoff_type = $config->handoff || ""
 	$handoff_code = $config->handoff_code || ""
-	panic(not strings.has_prefix($handoff_code, "/"))
+	panic($handoff_code != "" and not strings.has_prefix($handoff_code, "/"))
 
 	# This is a giant driver://user:password@host:port/whatever URL...
 	$bmc_uri = $config->bmc_uri || ""