METATADA: Fix unsetting "cloakhost"

Correctly re-generate the "cloaked hostname" when removing the
"cloakhost" using an empty string by passing down NULL instead of the
empty string, which results in protocol violations (for example on
WHOIS).

src/ngircd/irc-metadata.c

@@ -72,7 +72,9 @@ IRC_METADATA(CLIENT *Client, REQUEST *Req)
 	}
 
 	if (strcasecmp(Req->argv[1], "cloakhost") == 0) {
-		Client_UpdateCloakedHostname(target, prefix, Req->argv[2]);
+		/* Set or remove a "cloaked hostname". */
+		Client_UpdateCloakedHostname(target, prefix,
+					     *Req->argv[2] ? Req->argv[2] : NULL);
 		if (Client_Conn(target) > NONE && Client_HasMode(target, 'x'))
 			IRC_WriteStrClientPrefix(target, prefix,
 					RPL_HOSTHIDDEN_MSG, Client_ID(target),