Ensure that description is not null when parsing index.

At time of writing (and for some time before), fdroidserver has forced a description of "No description available" for apps which don't have descriptions at all:

• https://gitlab.com/fdroid/fdroidserver/blob/0.6.0/fdroidserver/metadata.py#L876

However, if the description is not set for whatever reason, it should not crash the client.

Identified in the now closed #739 (closed).