Fix logic for finding the beta version number

* autogen.sh: Check if $tmp is empty after each assignment.

This fixes the problem that due to the gpg4win-5.0.0-beta103 tag
./autogen.sh --find-version gpg4win 5 0 0 erroneously outputs
version 5.0.0 instead of 5.0.0-betaNNN.

Fix originally done for gpg4win and copied to gpgrt by wk@gnupg.org:

commit d3452493e665a4175218b7272ca8b1ca6fc57718
Author:     Ingo Klöcker <dev@ingo-kloecker.de>
AuthorDate: Wed Feb 19 15:46:55 2025 +0100