fix: handle null emails for leaderboard

[muhammad-ammar]

Description: Leaderboard API is crashing due to null emails. Changes in this PR will handle the null emails gracefully and show their aggregated data in table. Please see the screenshot below.
JIRA: https://2u-internal.atlassian.net/browse/ENT-9704

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.