ENT-9704: fix: Fixed null email issue for leaderboard.

[saleem-latif]

Jira Ticket: ENT-9704

Description:
This PR fixes a bug that was causing 500 error for leaderboard. It also implements a new feature where accumulated data for NULL emails is returned by the last page of the leaderboard API endpoint.

Merge checklist:

• Any new requirements are in the right place (do not manually modify the requirements/*.txt files)
  • base.in if needed in production but edx-analytics-data-api doesn't install it
  • test-master.in if edx-analytics-data-api pins it, with a matching version
  • make upgrade && make requirements have been run to regenerate requirements
• make static has been run to update webpack bundling if any static content was updated
• ./manage.py makemigrations has been run
  • Checkout the Database Migration Confluence page for helpful tips on creating migrations.
  • Note: This must be run if you modified any models.
    • It may or may not make a migration depending on exactly what you modified, but it should still be run.
  • This should be run from either a venv with all the edx-analytics-data-api requirements installed or if you checked out edx-enterprise-data into the src directory used by edx-analytics-data-api, you can run this command through an edx-analytics-data-api shell.
    • It would be ./manage.py makemigrations in the shell.
• Version bumped
• Changelog record added
• Translations updated (see docs/internationalization.rst but also this isn't blocking for merge atm)

Post merge:

• Tag pushed and a new version released
  • Note: Assets will be added automatically. You just need to provide a tag (should match your version number) and title and description.
• After versioned build finishes in Travis, verify version has been pushed to PyPI
  • Each step in the release build has a condition flag that checks if the rest of the steps are done and if so will deploy to PyPi.
    (so basically once your build finishes, after maybe a minute you should see the new version in PyPi automatically (on refresh))
• PR created in edx-analytics-data-api to upgrade dependencies (including edx-enterprise-data)
  • This must be done after the version is visible in PyPi as make upgrade in edx-analytics-data-api will look for the latest version in PyPi.
  • Note: the edx-enterprise-data constraint in edx-analytics-data-api must also be bumped to the latest version in PyPi.