[SYNPY-1417] Updates for the annotation model

[BryanFauble]

Problem:

1. When using the annotations field you had to first set it to a dict to use it since the default was None.
2. When using the annotations field it was not intuitive to set it to an empty dict to delete all annotations on an entity.

Solution:

1. Updating the default for the annotation field to be an empty dict.
2. Updating the logic around the annotation field such that if it is Empty or None it will be treated as a delete. It still means though that to delete annotations for an entity that is already stored in Synapse you'll need to do a "destructive" store (Meaning you .get() it then .store() it.

Testing:

1. Verified with new and existing integration tests