Fix Mixpanel distinct_id for anonymous events

[nunofgs]

This PR adds support for anonymous Mixpanel events by allowing the distinct_id property to be empty, as documented by Mixpanel:

If the event is not associated with any user, set distinct_id to the empty string. Events with an empty distinct_id will be excluded from all behavioral analysis.

Before this PR (when distinct_id is empty):

After this PR:

Testing

Include any additional information about the testing you have completed to
ensure your changes behave as expected. For a speedy review, please check
any of the tasks you completed below during your testing.

• Added unit tests for new functionality
• Tested end-to-end using the local server
• [If destination is already live] Tested for backward compatibility of destination. Note: New required fields are a breaking change.
• [Segmenters] Tested in the staging environment
• [Segmenters] [If applicable for this change] Tested for regression with Hadron.

[joe-ayoub-segment]

Hi @brennan - this is an external PR for an internally built Destination. Can you triage please?