Use more consistent set_key seq value when nothing is being set

Use NULL instead of (u8 *) "" as the seq value and make sure the
driver wrapper implementations can handle NULL value. This was
previously already done in number of places, but not everywhere.

wpa_supplicant/wpa_supplicant.c

@@ -134,7 +134,7 @@ int wpa_set_wep_keys(struct wpa_supplicant *wpa_s, struct wpa_ssid *ssid)
 		set = 1;
 		wpa_drv_set_key(wpa_s, WPA_ALG_WEP,
 				(u8 *) "\xff\xff\xff\xff\xff\xff",
-				i, i == ssid->wep_tx_keyidx, (u8 *) "", 0,
+				i, i == ssid->wep_tx_keyidx, NULL, 0,
 				ssid->wep_key[i], ssid->wep_key_len[i]);
 	}
 

wpa_supplicant/wpas_glue.c

@@ -212,7 +212,7 @@ static int wpa_eapol_set_wep_key(void *ctx, int unicast, int keyidx,
 	return wpa_drv_set_key(wpa_s, WPA_ALG_WEP,
 			       unicast ? wpa_s->bssid :
 			       (u8 *) "\xff\xff\xff\xff\xff\xff",
-			       keyidx, unicast, (u8 *) "", 0, key, keylen);
+			       keyidx, unicast, NULL, 0, key, keylen);
 }