fix(external course sync): sync course run dates if they are missing

[asadali145]

What are the relevant tickets?

Follow up fix for https://github.com/mitodl/mitxpro/pull/2998

Description (What does it do?)

Course run dates were not updating If an external course run ID is added for a run and dates are null but the API has dates.

How can this be tested?

• Checkout master branch
• Run command ./manage.py sync_external_course_runs --vendor-name emeritus
• Go to Django Admin, filter Emeritus course runs.
• Remove start and end date for a course run. The external course run ID should exist.
• Now run the management command again ./manage.py sync_external_course_runs --vendor-name emeritus
• The start and end date won't update.
• Checkout this branch and run the command again.
• Dates should sync.