Additional Software config app fails when virtual packages are listed in live-additional-software.conf

It fails with with "Failed to read additional software configuration 'NoneType' object has no attribute 'summary'".

Reported in wb:ce07b2e30c0252a0060cb8876736ef2b, I can reproduce. FWIW, it happened when the user started the app while asp-install was running. Reading the code, I understand this can only happen when the package's name exists in apt.cache.Cache(), the package is not installed, and apt_package.candidate is None, which is possible (https://apt-team.pages.debian.net/python-apt/library/apt.package.html#apt.package.Package.candidate). I think we should not spend time trying to understand in which exact situation this can happen, but instead handle the case when .candidate is None, and display a "[description not available]" placeholder.