fix bug in create lagged component names without target lags

[dennisbader]

Checklist before merging this PR:

• Mentioned all issues that this PR fixes or addresses.
• Summarized the updates of this PR under Summary.
• Added an entry under Unreleased in the Changelog.

Fixes #2574.

Summary

• Fixes a bug when using darts.utils.data.tabularization.create_lagged_component_names() with target lags=None, that did not return any lagged target label component names.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.87%. Comparing base (0b9efd0) to head (2f1d043).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2576      +/-   ##
==========================================
- Coverage   93.88%   93.87%   -0.02%     
==========================================
  Files         139      139              
  Lines       14862    14850      -12     
==========================================
- Hits        13953    13940      -13     
- Misses        909      910       +1     

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