Database
db={"cova_order":[{InStock:["1"],woo_order_id:3054,woo_order_data:["3054"]},{InStock:["2"],woo_order_id:"",woo_order_data:[]},{InStock:["3"],woo_order_id:null,woo_order_data:[]},{InStock:["4"],woo_order_data:[]}]}
Query
db.cova_order.aggregate([{$project:{InStock:1,woo_order_id:1,woo_order_data:1,cova_items:{$cond:[{$eq:[{"$ifNull":["$woo_order_id",""]},""]},"$InStock","$woo_order_data"]}}}])