lorawan: make possible to send empty frames

Empty frames are allowed by the LoRaWAN protocol and are actually useful
to open new RX slots or flush MAC commands in stack. Therefore allow the
data pointer to be NULL if len is 0.

Signed-off-by: Aurelien Jarno <[email protected]>

subsys/lorawan/lorawan.c

@@ -595,7 +595,7 @@ int lorawan_send(uint8_t port, uint8_t *data, uint8_t len,
 	int ret = 0;
 	bool empty_frame = false;
 
-	if (data == NULL) {
+	if (data == NULL && len > 0) {
 		return -EINVAL;
 	}