test: ensure notices generated do have content

[Hellgartner]

Summary of changes

Add a check in the build step to ensure that the notice document is not empty

Context and reason for change

There was a bug where we delivered an empty notices document
#2760

This shall prevent a repetition of the bug

Alternative Solutions considered:

• Write a playwright based e2e test: Did not fly as playwright does not manage to follow the "jump" from electron to the real browser after clicking the notice menu link.
• Writing jest unit test would have required translating the tooling to typescript (jest support for mjs -- or to be exact the used include statements -- is currently only experimental) which was determined to be out of scope for this ticket

How can the changes be tested

Break the notice generation (e.g. by modifying generateNotices.mjs) and observe yarn build (or yarn generate-notice) to fail.

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