Correctly treat empty modules as blackboxes in Verific

Signed-off-by: Clifford Wolf <clifford@clifford.at>

frontends/verific/verific.cc

diff --git a/frontends/verific/verific.cc b/frontends/verific/verific.cc
index c2086af..de41e1a 100644
--- a/frontends/verific/verific.cc
+++ b/frontends/verific/verific.cc
@@ -130,7 +130,7 @@
 
 bool is_blackbox(Netlist *nl)
 {
-	if (nl->IsBlackBox())
+	if (nl->IsBlackBox() || nl->IsEmptyBox())
 		return true;
 
 	const char *attr = nl->GetAttValue("blackbox");