pfdenied: fix checking root anchor

pfctl doesn't like empty anchors (-a ''), but we can specify the root
anchor as '/' too, so do that instead.

PR:		295324
Tested by:	Paweł Krawczyk
MFC after:	1 week
Sponsored by:	Rubicon Communications, LLC ("Netgate")

usr.sbin/periodic/etc/security/520.pfdenied

@@ -41,7 +41,7 @@ rc=0
 if check_yesno_period security_status_pfdenied_enable
 then
 	TMP=`mktemp -t security`
-	for _a in "" $(pfctl -a "blacklistd" -sA 2>/dev/null) $(pfctl -a "blocklistd" -sA 2>/dev/null) ${security_status_pfdenied_additionalanchors}
+	for _a in "/" $(pfctl -a "blacklistd" -sA 2>/dev/null) $(pfctl -a "blocklistd" -sA 2>/dev/null) ${security_status_pfdenied_additionalanchors}
 	do
 		pfctl -a "${_a}" -sr -v -z 2>/dev/null | \
 		nawk '{if (/^block/) {buf=$0; getline; gsub(" +"," ",$0); if ($5 > 0) print buf$0;} }' >> ${TMP}