device_dt_metadata: handle dt_meta being NULL

Using  DEVICE_DEFINE, a device without a corresponding DT node can be
defined (for example CRYPTO_MTLS), Z_DEVICE_INIT() does not initialize
dt_meta for such devices, leaving the field as NULL.
device_get_dt_nodelabels() and functions calling it have to handle
dev->dt_meta == NULL to prevent fatal errors.

Signed-off-by: Jan Peters <peters@kt-elektronik.de>