Lazy Reference to Abstract class attempts to obtain the classes `id_field`

This is a similar issue to #837, though pertaining to LazyReferenceFields as opposed to ReferenceFields

The following code should enable you to replicate the issue in mongoengine version 0.18.0

import mongoengine as me

class AbstractExample(me.Document):
    meta = {"abstract": True}

class ConcreteExample(AbstractExample):
    pass

class Referrer(me.Document):
    reference = me.LazyReferenceField(AbstractExample)

Saving a Referrer to the database will trigger a KeyError: id_field.
Replacing the LazyReferenceField with a regular ReferenceField doesn't trigger this exception.

Start by running the provided reproduction against mongoengine 0.18.0 and trace the LazyReferenceField save path for an abstract document. Confirm the failure is the KeyError: id_field, then add a regression test showing that saving Referrer with AbstractExample no longer raises it.