automake: fall back gracefully when texinfo inputs don't exist

automake: fall back gracefully when texinfo inputs don't exist

Fixes automake bug https://bugs.gnu.org/54063.

The function scanning for @setfilename will fall back to a default
value if the input doesn't have one defined.  But we need to handle
the case where the file doesn't even exist before falling back.

* bin/automake.in: Scan /dev/null for @setfilename if input doesn't exist.
* t/list-of-tests.mk: Add txinfo-no-setfilename-no-inputs.sh.
* t/txinfo-no-setfilename-no-inputs.sh: New test.