fix: Last Influx data updated at never updates

[novakzaballa]

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Changes

• Added a filter for when the "influx update at" field is null.

How did you test this code?

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[gagantrivedi]

If we are only fetching the first element, we can do desc(null_last=True) /
docs: https://docs.djangoproject.com/en/3.1/ref/models/expressions/#django.db.models.Expression.desc

[novakzaballa]

Done.

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (e62be85) 95.53% compared to head (b420214) 95.53%.

❗ Current head b420214 differs from pull request most recent head faec4e8. Consider uploading reports for the commit faec4e8 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2802   +/-   ##
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  Coverage   95.53%   95.53%           
=======================================
  Files         994      994           
  Lines       28072    28072           
=======================================
  Hits        26818    26818           
  Misses       1254     1254           

Files	Coverage Δ
api/sales_dashboard/views.py	61.29% <ø> (ø)

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