Checkmk Werk 13677: mk_apt: Check crashes on single-line agent sections

ID: 13677 Title: mk_apt: Check crashes on single-line agent sections Component: Checks & agents Level: 1 Class: Bug fix Version: 2.2.0i1 This fixes a regression that got introduced with Werk #13578. While the above mentioned Werk fixed an issue with ignored security updates for the "APT normal and security updates" plugin, it made a wrong assumption to the output of the underlying <tt>apt-get</tt> command, that lead to a crash for outputs that exactly consist of one security update item.