PopDB API returns error if site is empty/has never been accessed

[nikmagini]

Currently popdb APIs return an error "Error occurred during sitename validation, cause: param must be a valid site name" if the site name is valid but the site has never been accessed. Example T2_MY_UPM_BIRUNI

https://cmsweb.cern.ch/popdb/popularity/DSStatInTimeWindow/?sitename=T2_MY_UPM_BIRUNI

It makes more sense to return an empty result set in this case instead