[FIX] tableview: Fix errors with sparse basket columns

[ales-erjavec]

Issue

Since #6370 errors appear in data table when a sparse basket column in on input.

When clicking 'Show variable labels':

Traceback (most recent call last):
  File "/home/ales/devel/orange3/Orange/widgets/data/utils/tableview.py", line 175, in __headerDataChanged
    text += "\n".join(key for key, _ in items)
  File "/home/ales/devel/orange3/Orange/widgets/data/utils/tableview.py", line 175, in <genexpr>
    text += "\n".join(key for key, _ in items)
ValueError: not enough values to unpack (expected 2, got 1)

When trying to sort a basket column:

Traceback (most recent call last):
  File "/home/ales/devel/orange3/Orange/widgets/data/owtable.py", line 341, in _on_sort_indicator_changed
    self.stored_sort.append((var.name, order))
AttributeError: 'NoneType' object has no attribute 'name'

Description of changes

Fix errors in handling sparse basket column in tableview.

Includes

• Code changes
• Tests
• Documentation

[VesnaT]

I can temporarily merge this to avoid the crash, but I don't agree with the solution.
The table could be sortable by Y or metas even though it has sparse X.

[markotoplak]

To me, this seems like removing a feature (being able to sort such table on metas) for the occasional, user-avoidable :) bug. In such cases, I prefer buggy features to no features.

[codecov]

Codecov Report

Merging #6409 (68d95b0) into master (1056fc3) will increase coverage by 0.01%.
The diff coverage is 87.87%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6409      +/-   ##
==========================================
+ Coverage   87.61%   87.62%   +0.01%     
==========================================
  Files         321      321              
  Lines       69024    69056      +32     
==========================================
+ Hits        60476    60511      +35     
+ Misses       8548     8545       -3