When rendering templates for packages, be more forgiving about missing
metadata. For some repository types - like maven - metadata is uploaded
separately. If that upload fails, or does not happen, there will be no
metadata.
In that case, Forgejo should handle it gracefully, and render as much of
the information as possible, without erroring out. Rendering without
metadata allows one to delete a partial package, while if we throw
errors, that becomes a whole lot harder.
This patch adjusts the generic metadata template, and also the maven
template. There may be more cases of the same problem lying around.
Fixes#3663.
Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>
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