top_viral_desc for custom DB/taxless DB is NA. Should be more informative

[thackl]

For top_viral_desc, use top VDB description as fallback if no viral hits are available, e.g. if the custom DB has no taxonomy info.

[thackl]

@nbrait This should also be fixed now, but I didn't have a good example to test it. Could you verify that this now works and close this issue afterwards?

[nbrait]

works well.