fix: validate key before proceeding with deep assignment

[mmaietta]

https://github.com/electron-userland/electron-builder/security/code-scanning/9
https://github.com/electron-userland/electron-builder/security/code-scanning/10
https://github.com/electron-userland/electron-builder/security/code-scanning/11
https://github.com/electron-userland/electron-builder/security/code-scanning/12

[changeset-bot]

🦋 Changeset detected

Latest commit: d3a26bf

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 10 packages

Name	Type
builder-util	Patch
app-builder-lib	Patch
dmg-builder	Patch
electron-builder-squirrel-windows	Patch
electron-builder	Patch
electron-publish	Patch
electron-forge-maker-appimage	Patch
electron-forge-maker-nsis-web	Patch
electron-forge-maker-nsis	Patch
electron-forge-maker-snap	Patch

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