[admin] Avoid setting default extra_context to mutable object
[admin] Avoid setting default extra_context to mutable object

The method definition was flawed.
It must be:
    `extra_context=None`
instead of:
    `extra_context={}`
because the method definition is evaluated only
once at import time, setting a default mutable object
causes the object be kept in memory and passed to
subsequent requests.

netjson/django-netjsongraph,netjson/django-netjsongraph,interop-dev/django-netjsongraph,netjson/django-netjsongraph,netjson/django-netjsongraph,interop-dev/django-netjsongraph,interop-dev/django-netjsongraph