TreeViewer: Escape class values

[janezd]

Issue

Fixes #7004.

@wvdvegte noticed that "Tree viewer doesn't show first category of discretized numeric variable", which is true, and also interestingly misleading. :) I assumed that it has something to do with the index 0 being falsy, while in fact the problem was that the label of the first value of categorized variable starts with a character <, and we output this as HTML.

Description of changes

Escape the label.

Includes

• Code changes
• Tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.77%. Comparing base (5eb97b8) to head (7ceb2e7).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7124   +/-   ##
=======================================
  Coverage   88.77%   88.77%           
=======================================
  Files         334      334           
  Lines       73671    73671           
=======================================
  Hits        65402    65402           
  Misses       8269     8269           

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