In order to file jointly, the OP must first determine the spouse's US tax status because different elections may be required.

US TIN is required of the spouse for MFJ.  It is not permissible to leave that blank or indicate "Not Applicable" (btw, there's no option for "None").  US TIN is also required if spousal exemption is to be claimed (although it's not relevant under TCJA).

Not as relevant in this case but it would not be advisable to elect joint filing with NRA spouse without first finding out whether certain joint election had previously been made by either spouse, understanding the alien spouse's income and net worth, now and in the foreseeable future, and discussing with the client the pros and cons, including any potential implications downstream.  Mistake could prove very costly.