Fix HoloViews.layout issue

[MarcSkovMadsen]

Fixes HoloViews.layout issue described in #5628 (comment), i.e. that the widget_layout is shown when it holds no widgets.

Todo.

• Add test cases
• Get Philipp to review test cases
• Find issue or place where fix should be implemented.
• Implement solution

[MarcSkovMadsen]

Please review the below test cases. Thanks.

If you agree that the layout should not contain the widget_layout when its empty. Then we should try to find fix.

The alternative would be to set visible=False dynamically on the widget_layout when it contains no objects.

If you know where the fix should be applied I would appreciate you tell me or implement the fix. I have spent some time without being able to identify the place. Thanks.

UPDATE: LOOKING AT THE HoloViews implementation I can see that I can't expect the widget layout to be in position 1. But I still need to know whether the layout objects should be updated or the widget_layout invisible.

[codecov]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (1f049f0) 84.62% compared to head (9a96488) 40.64%.

Files	Patch %	Lines
panel/tests/pane/test_holoviews.py	42.85%	12 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6063       +/-   ##
===========================================
- Coverage   84.62%   40.64%   -43.99%     
===========================================
  Files         295      295               
  Lines       43976    43983        +7     
===========================================
- Hits        37215    17877    -19338     
- Misses       6761    26106    +19345     

Flag	Coverage Δ
ui-tests	40.64% <42.85%> (+<0.01%)	⬆️
unitexamples-tests	?

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