[server] Fix case where empty array passed to fetchKnownUserInfos throws exception
[server] Fix case where empty array passed to fetchKnownUserInfos throws exception

It results in a SQL error around `IN ()`

Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal