[#820] Fix exception in ProjectUpdateResourceExtra.dehydrate()

primary_location_data_for_update() in
ProjectUpdateResourceExtra.dehydrate() throws an unhandled exceltion if
the organisation or project doesn't have a primary location.

Since we don't require orgs to have a location wrap the creation of the
primary_location dics in a try-except and return None if there's no
location.
Merge pull request #821 from akvo/feature/820_fix_updates_extra

Looks good