[DOP-13681] Allow empty pipeline in MongoDB.pipeline

[dolfinus]

Change Summary

Sometimes it is handy to just fetch some data from MongoDB to see infered schema. But MongoDB.pipeline requires to pass pipeline={...} argument, so users have to pass here some nonsense to bypass this.

Now this argument is optional.

Related issue number

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.39%. Comparing base (02ba65a) to head (85e64eb).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #237   +/-   ##
========================================
  Coverage    94.39%   94.39%           
========================================
  Files          204      204           
  Lines         7880     7882    +2     
  Branches      1401     1403    +2     
========================================
+ Hits          7438     7440    +2     
  Misses         317      317           
  Partials       125      125           

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