You can send something if you want to or you can write that you don't have one or you can write N/A on the income requirements. The last option was actually what a woman in your situation was advised to do by CIC. I haven't seen her for a while so I guess her sponsorship probably came through and she is now living happily ever after. I hope so anyway. You don't have to prove income to sponsor a spouse, that is basically why income is not relevant.
