Fixes and guards for rendering ChatFeed and Card

[philippjfr]

• Ensure Theme does not error out if a child of a layout has not yet initialized a model
• Do not re-render Card superfluously, which could cause race conditions and rendering issues
• Ensure that ChatFeed reflects the model of layout it is rendering to ensure that components that assume a component has a corresponding model will find one.

Fixes #6140

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (d40dd3b) 84.67% compared to head (9632f4d) 84.59%.

Files	Patch %	Lines
panel/chat/feed.py	0.00%	4 Missing ⚠️
panel/theme/base.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6177      +/-   ##
==========================================
- Coverage   84.67%   84.59%   -0.08%     
==========================================
  Files         296      296              
  Lines       44224    44229       +5     
==========================================
- Hits        37445    37415      -30     
- Misses       6779     6814      +35     

Flag	Coverage Δ
ui-tests	40.71% <0.00%> (-0.02%)	⬇️
unitexamples-tests	72.51% <16.66%> (-0.07%)	⬇️

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