Fix issue with adding pending writes

[philippjfr]

Small oversight broke HoloViews tests: https://github.com/holoviz/holoviews/actions/runs/8566521576/job/23476483126?pr=6174#step:8:416

[codecov]

Codecov Report

Attention: Patch coverage is 10.00000% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 82.80%. Comparing base (caf0622) to head (efa42bc).

Files	Patch %	Lines
panel/io/document.py	10.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6680      +/-   ##
==========================================
- Coverage   82.83%   82.80%   -0.04%     
==========================================
  Files         313      313              
  Lines       46160    46164       +4     
==========================================
- Hits        38238    38227      -11     
- Misses       7922     7937      +15     

Flag	Coverage Δ
ui-tests	39.98% <10.00%> (-0.01%)	⬇️
unitexamples-tests	71.08% <10.00%> (-0.03%)	⬇️

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