Fix Issue #7299 Check for area property before use

[ohfuchs]

Added check to prevent notice when there is no template_part area defined.

for example an area property is missing Mini Cart template part

Fixes #7299

[sunyatasattva]

Hello @ohfuchs ! Thank you for the contribution: it looks good! I would ask you however to improve a bit the PR description. You should use the template we offer whenever you open a PR, there are important things such as the testing steps and even the changelog entry that we use while deploying new versions, so these are crucial to us.

On top of that, a good PR description is extremely helpful for historical reasons, as we are many people working on this project and it helps being able to go back on a line of code and see a complete description and reasoning on why it was added.

That would help very much!

[github-actions]

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[albarin]

Fixed by #8443