[server] Treat empty string sessionID or cookie as null
[server] Treat empty string sessionID or cookie as null

Necessary since `FormData` doesn't accept `null`, so we need another way to communicate that the `sessionID` or `cookie` ought to be set but isn't.

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