Fix serving of global template in notebook

[philippjfr]

Fixes a regression introduced in the layout builder PR.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (b0b94ad) 82.69% compared to head (ccc91f8) 82.55%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
panel/io/handlers.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6210      +/-   ##
==========================================
- Coverage   82.69%   82.55%   -0.14%     
==========================================
  Files         301      301              
  Lines       45099    45099              
==========================================
- Hits        37293    37231      -62     
- Misses       7806     7868      +62     

Flag	Coverage Δ
ui-tests	38.47% <0.00%> (ø)
unitexamples-tests	71.65% <0.00%> (-0.19%)	⬇️

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