arrayToObject should not error out on explicit value of null for value

If my object is o:{ "foo": null } and {$objectToArray:"$o"} converts it to {k:"foo",v:null} then $arrayToObject must be able to convert it back to the object, but instead it gives the error:

Error: command failed: {

	"ok" : 0,

	"errmsg" : "$arrayToObject requires an object with keys 'k' and 'v'. Missing either or both keys from: {k: \"comment\", v: null}",

	"code" : 40393,

	"codeName" : "Location40393"

}

Explicit null is a different type than missing, so we're probably checking for "nullish" when we should be checking for "missing".