all: Use NULL instead of "" when calling mp_raise exception helpers.

This is the established way of doing it and reduces code size by a little
bit.

ports/esp8266/machine_hspi.c

@@ -149,7 +149,7 @@ mp_obj_t machine_hspi_make_new(const mp_obj_type_t *type, size_t n_args, size_t
     // args[0] holds the id of the peripheral
     if (args[0] != MP_OBJ_NEW_SMALL_INT(1)) {
         // FlashROM is on SPI0, so far we don't support its usage
-        mp_raise_ValueError("");
+        mp_raise_ValueError(NULL);
     }
 
     machine_hspi_obj_t *self = m_new_obj(machine_hspi_obj_t);