[{name:"sunil",tags:null},{name:"radha",tags:[{k:"max",v:"4"},{k:"min",v:"3"}]}]

db.collection.update({},[{$set:{tags:{$cond:{if:{$in:[{$type:"$tags"},["null","missing","undefined"]]},then:[{k:"new k",v:"new v"}],else:{$concatArrays:["$tags",[{k:"new k",v:"new v"}]]}}}}}],{multi:true})