Add-on store: handle description from manifest is None (#15245)
None
Summary of the issue:
The description fields in the manifest are optional, and default to None.
This is not handled by the add-on models correctly, which expect the description field to always be a string
Description of user facing changes
Add-ons without descriptions can be correctly viewed in the add-on store
Description of development approach
return an empty string if there is no description for the add-on.
