[FIX] oweditdomain: Fix an IndexError when all rows are deselected

[ales-erjavec]

Issue

'Edit Domain' widget raises a:

---------------------------- IndexError Exception -----------------------------
Traceback (most recent call last):
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/data/oweditdomain.py", line 425, in on_value_selection_changed
    i = self.values_edit.selectionModel().selectedRows()[0].row()
IndexError: list index out of range
-------------------------------------------------------------------------------

... when editing a discrete variable and the current item in the 'Values' list view is deselected (via a Ctrl + mouse click or when selecting a different variable to edit in the left 'Domain Features' view.

Description of changes

Check for an empty selection.

Includes

• Code changes
• Tests
• Documentation

[codecov-io]

Codecov Report

Merging #3183 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3183      +/-   ##
==========================================
+ Coverage   82.63%   82.64%   +<.01%     
==========================================
  Files         340      340              
  Lines       58790    58802      +12     
==========================================
+ Hits        48579    48594      +15     
+ Misses      10211    10208       -3