build-export: Fix use of uninitialized memory

This fixes build-export so that it doesn't try to use uninitialized
memory in a timestamp object when creating appstream data. Before the
--timestamp option existed, a timestamp of 0 was used, so fall back to
that. Also, update a relevant comment.

Closes: #1408
Approved by: alexlarsson
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f