Per the CBSA:

Empty conveyances in the air and marine modes of transport are currently exempt from transmitting pre-arrival/pre-load data; therefore, they are exempt from transmitting the CACM. 

However, if a carrier (conveyance owner/operator) choses to voluntarily transmit an electronic ACI conveyance report for an ACI exempt conveyance, the CACM must be transmitted in order for the arrival status in the CBSA system to be finalized.