change BatchSendingsItemsEndpoint.import method return type to void

[PavelVais]

Description

Endpoint /api/batch-sendings/:id/items/import returns empty body and there is no plan to change it anymore in the near future

Motivation and context

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• New feature (non-breaking change which adds functionality)
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.52%. Comparing base (19f5cd2) to head (531bae2).

Additional details and impacted files

@@            Coverage Diff            @@
##                2.x     #257   +/-   ##
=========================================
  Coverage     97.52%   97.52%           
  Complexity      467      467           
=========================================
  Files            74       74           
  Lines          1089     1089           
=========================================
  Hits           1062     1062           
  Misses           27       27           

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