popdbcombine(lastAcc) APIs return proxy error if sitename is not passed

[nikmagini]

The popdbcombine/popdbcombineLastAcc APIs return a '502 proxy error' after a long time when the 'sitename' argument is omitted, for example:

https://cmsweb-testbed.cern.ch/popdb/victorinterface/popdbcombineLastAcc/

Proxy Error

The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request GET /auth/complete/popdb/victorinterface/popdbcombineLastAcc/.

Reason: Error reading from remote server

Looking at the code, it seems that sitename defaults to 'T%' when omitted, causing a large query on popdb and an error from the PhEDEx datasvc. Recommending to make the 'sitename' argument mandatory instead, and return error immediately.