Add client_session context entity if anonymous tracking with session tracking is enabled

[matus-tomlein]

Is your feature request related to a problem? Please describe.
In case one creates tracker with the following configuration:

snowplow('newTracker', 'sp', '{{collector_url_here}}', {
  ...
  anonymousTracking: { withSessionTracking: true },
  contexts: {
    session: true
  }
);

The session context entity is not added to events even though sessions are tracked.

This is because of the check on this line which doesn't allow the session entity to be added in case anonymous tracking is enabled.

Describe the solution you'd like
Session entity should be added if anonymous tracking with session tracking is enabled. However, the userId property in the entity should be a null UUID: 00000000-0000-0000-0000-000000000000