Add counter for missing course ID in user activity.

[doctoryes]

Analytics Pipeline Pull Request

Make sure that the following steps are done before merging:

• [ ] If you have a migration please contact data engineering team before merging.
• [ ] Before merging run full acceptance tests suite and provide URL for the acceptance tests run.
• [ ] A member of data engineering team has approved the pull request.

[codecov[bot]]

Codecov Report

Merging #848 (294934d) into master (5d94516) will decrease coverage by 0.00%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #848      +/-   ##
==========================================
- Coverage   74.03%   74.02%   -0.01%     
==========================================
  Files         208      208              
  Lines       23886    23887       +1     
==========================================
  Hits        17683    17683              
- Misses       6203     6204       +1     

Impacted Files	Coverage Δ
edx/analytics/tasks/insights/user_activity.py	74.46% <0.00%> (-0.54%)	:arrow_down:

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