[MNT] WidgetTest: Add tests on nasty data

[janezd]

Issue

Widget are supposed to survive data all-nans or without rows, attributes, classes... Many don't because we don't test it.

Also fixes #7151 (via https://github.com/biolab/orange3/pull/6223/files#diff-55f7ddbd86ca693bb61aeac9378a53ae12ba29aecf48fc73f7ebe012a36a7ebaR402).

Description of changes

WidgetTest, the base class for all widget tests now adds a test that sends nasty data to all inputs that accept Table.

PR adds various fixes to widgets so they pass the tests.

Includes

• Code changes
• Tests

[codecov]

Codecov Report

❌ Patch coverage is 93.93939% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.02%. Comparing base (c807a41) to head (a15eac3).
⚠️ Report is 20 commits behind head on master.

❌ Your patch check has failed because the patch coverage (93.93%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6223      +/-   ##
==========================================
+ Coverage   88.91%   89.02%   +0.10%     
==========================================
  Files         335      335              
  Lines       73978    74086     +108     
==========================================
+ Hits        65779    65953     +174     
+ Misses       8199     8133      -66     

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