MiddlewareEvents model should allow nullable context

[lokhman]

Currently, user and url proxy fields in MiddlewareEvents are non-nullable, which makes the admin page display only those events that have both values in the context metadata.

The counter at the bottom page of the change list view displays the correct number of entries though, which makes this issue a bug.

We need to add null=True to both fields to display all events as expected for this model.