E.g

type Obj struct {
    SomeId objectid.ObjectID `bson:"listId,omitempty" json:"someId,omitempty"`
}

When an object is marshalled with SomeId ==  primitive.NilObjectID and omitempty tag, it shouldn't be included in the marshalled string.

Now it's marshalled into

{
   "someId: "00000000000000000000000"
}