Run unbound-anchor when root.key is empty, not just when it is absent.

PR:		232555
Submitted by:	Ari Suutari <ari@stonepile.fi>
MFC after:	3 days

libexec/rc/rc.d/local_unbound

@@ -92,7 +92,7 @@ local_unbound_prestart()
 	fi
 
 	# Retrieve DNSSEC root key
-	if [ ! -f ${local_unbound_anchor} ] ; then
+	if [ ! -s ${local_unbound_anchor} ] ; then
 		run_rc_command anchor
 	fi
 }