Fix crash when serializing empty shadow tree. #29882

	name: manifest
	on:
	push:
	branches:
	- master
	pull_request:
	paths:
	- 'tools/**'
	jobs:
	build-and-tag:
	runs-on: ubuntu-20.04
	steps:
	- name: Set up Python
	uses: actions/setup-python@v5
	with:
	python-version: '3.12'
	- name: Checkout
	uses: actions/checkout@v4
	with:
	fetch-depth: 50
	- name: Install dependencies
	run: \|
	sudo apt-get -qqy install zstd
	pip install -r tools/wpt/requirements.txt
	- name: Run manifest_build.py
	# Use a conditional step instead of a conditional job to work around #20700.
	if: github.repository == 'web-platform-tests/wpt'
	run: tools/docker/retry.py --delay 60 python tools/ci/manifest_build.py
	env:
	GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

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