Fix transformer

[egillax]

Transformer was failing when there were no numerical features. Fixed and added a unit test.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (2d8f9af) 99.59% compared to head (46c3213) 99.59%.

❗ Current head 46c3213 differs from pull request most recent head 916c32c. Consider uploading reports for the commit 916c32c to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           develop      #93   +/-   ##
========================================
  Coverage    99.59%   99.59%           
========================================
  Files            9        9           
  Lines          496      496           
========================================
  Hits           494      494           
  Misses           2        2           

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