Allow empty archives

Previously, when an archive was empty, it would cause Make to endlessly
restart. This was especially a problem for archives generated from
includes, as if the include was missing (or an empty folder), the
archive would have no members and Make would get stuck in a retry loop.

This commit provides the necessary adjustments to ensure archive code
continues working even when the archive is empty: The archive index
file is always written, and the archive files folder is made to exist
before attempting to `cd` into it.