[FIX] Select Rows: None on output when no data

[jerneju]

Issue

Rank widget crashes because receives empty Table and empty Domain which is created by Select Rows.

Description of changes

Select Rows commits None.

Includes

• Code changes
• Tests
• Documentation

[jerneju]

https://sentry.io/biolab/orange3/issues/386054859/

[codecov-io]

Codecov Report

Merging #2726 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2726      +/-   ##
==========================================
+ Coverage   76.03%   76.04%   +0.01%     
==========================================
  Files         338      338              
  Lines       59628    59641      +13     
==========================================
+ Hits        45339    45356      +17     
+ Misses      14289    14285       -4

[janezd]

A comment for other potential reviewers: it is not clear what widget should output in this case. @jerneju proposed None and I agreed. It makes sense from the user perspective: with None, the canvas shows that the widgets hasn't output anything. Otherwise, the canvas indicates that the widget has output something ... but this something is empty, which is a confusing non-difference for the user.

None is also better technically since most widgets do not expect empty tables and, worse, empty domains on inputs. (This is not to say that a well-behaved widget should not crash if this happens.)

[janezd]

I here agree with @astaric's usual comment about my code. If-else expressions in Python are mostly unreadable. I'd prefer

if output is not None and len(output):
    return output

You don't even need return None here, but you may add it to make it explicit.

[janezd]

Oh, besides, I don't like the function at all. I'd prefer having

if not len(matching_output):
    matching_output = None

and the same for non_matching_output.

The same amount of code, but way more explicit.

[janezd]

I made the change. Another option is to allow empty tables, but not empty domains. In this case we don't call remover if not len(data).