With the new localization support, the name/summary/description in the
metadata file becomes the global override. So most apps are not going to
have those fields present in their metadata file. This fixes the index.xml
generation to fall back to the localized versions of those fields when they
are not set in the metadata field.
https://forum.f-droid.org/t/what-has-happend-to-osmand
`fdroid update` crashed for apps that only had screenshots but no graphics
or localized texts because destdir was not being set in that case. This
fixes that and adds a test case.
closes#320!286
