[FIX] owpredictions: Fix a type error in report when using NoopDelegate

[ales-erjavec]

Issue

Fix an error when reporting regression model with data that does not have the target column:

-------------------------- AttributeError Exception ---------------------------
Traceback (most recent call last):
  File "/home/ales/devel/orange-widget-base/orangewidget/report/report.py", line 59, in show_report
    self.create_report_html()
  File "/home/ales/devel/orange-widget-base/orangewidget/report/report.py", line 98, in create_report_html
    self.send_report()
  File "/home/ales/devel/orange3/Orange/widgets/evaluate/owpredictions.py", line 930, in send_report
    self.report_table("Data & Predictions", merge_data_with_predictions(),
  File "/home/ales/devel/orange-widget-base/orangewidget/report/report.py", line 340, in report_table
    body = report_list(table, header_rows, header_columns)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/ales/devel/orange-widget-base/orangewidget/report/report.py", line 324, in report_list
    return join("  <tr>\n    {}</tr>\n".format(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/ales/devel/orange-widget-base/orangewidget/report/report.py", line 324, in <genexpr>
    return join("  <tr>\n    {}</tr>\n".format(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/ales/devel/orange3/Orange/widgets/evaluate/owpredictions.py", line 907, in merge_data_with_predictions
    [delegate.displayText(
  File "/home/ales/devel/orange3/Orange/widgets/evaluate/owpredictions.py", line 907, in <listcomp>
    [delegate.displayText(
     ^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoopItemDelegate' object has no attribute 'displayText'
-------------------------------------------------------------------------------

Description of changes

Fix type of NoopDelegate

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #6537 (cd67c66) into master (49dc3fd) will decrease coverage by 0.01%.
Report is 12 commits behind head on master.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6537      +/-   ##
==========================================
- Coverage   87.68%   87.68%   -0.01%     
==========================================
  Files         321      321              
  Lines       69371    69404      +33     
==========================================
+ Hits        60830    60857      +27     
- Misses       8541     8547       +6