Database
[{"mfe_average":[[true,5.352702824879613],[false,3.2361364317753383],[null,2.675027181819201]]}]
Query
db.collection.aggregate([{$set:{mfe_average:{$map:{input:"$mfe_average",as:"avg",in:{$concatArrays:[[{$cond:{if:{$eq:[{$arrayElemAt:["$$avg",0]},true]},then:"Valid",else:"Invalid"}}],{$slice:["$$avg",{$multiply:[{$subtract:[{$size:"$$avg"},1]},-1]}]}]}}}}}])