[FIX] Heatmap: Fix crash on data with empty columns

[jerneju]

Issue

Invalid number of variable columns
Columns in a domain and in a table do not match.
https://sentry.io/biolab/orange3/issues/196744285/

Description of changes

If a column in a table is dropped, it is erased in a domain as well.

Includes

• Code changes
• Tests
• Documentation

[jerneju]

https://sentry.io/biolab/orange3/issues/196744285/

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@34b5e94). Click here to learn what that means.
The diff coverage is 88.88%.

@@           Coverage Diff            @@
##             master   #2057   +/-   ##
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  Coverage          ?   70.7%           
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  Files             ?     315           
  Lines             ?   53905           
  Branches          ?       0           
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  Hits              ?   38114           
  Misses            ?   15791           
  Partials          ?       0

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[jerneju]

https://sentry.io/biolab/orange3/issues/196743618/

[janezd]

@jerneju, as a general comment: this PR contains a fix and a test, and some code reformatting. Try keeping these changes in separate commits so they are easier to review. That is, put the reformatting into a separate commit.

(My pull requests may not be, or at least haven't been in the past, the best examples to follow, though. :)