Make dag_run_id nullable in openapi.yaml

[jdddog]

A fix for the error:

observatory.api.client.exceptions.ApiTypeError: Invalid type for variable 'dag_run_id'. Required value type is str and passed type was NoneType at ['received_data'][0]['dag_run_id']

[keegansmith21]

Thanks for getting on to this Jamie!

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.06% 🎉

Comparison is base (af7b83c) 86.61% compared to head (70e9ce5) 86.68%.
Report is 1 commits behind head on develop.

❗ Current head 70e9ce5 differs from pull request most recent head 65b48c9. Consider uploading reports for the commit 65b48c9 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #631      +/-   ##
===========================================
+ Coverage    86.61%   86.68%   +0.06%     
===========================================
  Files           34       34              
  Lines         4626     4626              
  Branches       559      559              
===========================================
+ Hits          4007     4010       +3     
+ Misses         505      502       -3     
  Partials       114      114              

Files Changed	Coverage Δ
...pi/observatory/api/client/model/dataset_release.py	85.29% <ø> (ø)

... and 1 file with indirect coverage changes

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