sd-pppoe: use strna() instead of strempty()

In cases where we use for instance. log_debug() with a statement + string
it is better to output (Service-name: N/A) instead of (Service-name: )

src/libsystemd-network/sd-pppoe.c

@@ -385,7 +385,7 @@ static int pppoe_send_initiation(sd_pppoe *ppp) {
                 return r;
 
         log_debug("PPPoE: sent DISCOVER (Service-Name: %s)",
-                  strempty(ppp->service_name));
+                  strna(ppp->service_name));
 
         pppoe_arm_timeout(ppp);