Set drop = TRUE in split.shapviz()

[mayer79]

Splitting on an empty factor level does not work with split.shapviz(). Setting drop = TRUE` to fix the problem.

[codecov-commenter]

Codecov Report

Merging #117 (d4695a7) into main (3007aae) will not change coverage.
The diff coverage is 100.00%.

❗ Current head d4695a7 differs from pull request most recent head 5458cf6. Consider uploading reports for the commit 5458cf6 to get more accurate results

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@@           Coverage Diff           @@
##             main     #117   +/-   ##
=======================================
  Coverage   97.36%   97.36%           
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  Files          11       11           
  Lines         798      798           
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  Hits          777      777           
  Misses         21       21           

Files	Coverage Δ
R/methods.R	97.75% <100.00%> (ø)