[FIX] owdiscretize: Fix formatting display string when user role is undefined

[ales-erjavec]

Issue

Fix an error in display string formatting when updating widget input while a filter/search string is entered in the list view. I.e.

• File -> Discretize,
• Enter some text in the filter edit line
• Reload the file.

----------------------------- TypeError Exception -----------------------------
Traceback (most recent call last):
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/data/owdiscretize.py", line 345, in data
    hint, points, values = index.data(Qt.UserRole)
    ^^^^^^^^^^^^^^^^^^^^
TypeError: cannot unpack non-iterable NoneType object
-------------------------------------------------------------------------------

Description of changes

Fix formatting display string when user role is undefined

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #6498 (f746f36) into master (ff152a9) will decrease coverage by 0.01%.
The diff coverage is 85.71%.

❗ Current head f746f36 differs from pull request most recent head a8cf0c5. Consider uploading reports for the commit a8cf0c5 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6498      +/-   ##
==========================================
- Coverage   87.66%   87.66%   -0.01%     
==========================================
  Files         321      321              
  Lines       69374    69379       +5     
==========================================
+ Hits        60817    60820       +3     
- Misses       8557     8559       +2