partial fix for #1933

If the nickname must equal the account name (because always-on or
force-nick-equals-account), the correct error response to an empty
or otherwise invalid nickname is the usual "You must use your account
name as your nickname".

irc/client_lookup_set.go

@@ -117,7 +117,7 @@ func (clients *ClientManager) SetNick(client *Client, session *Session, newNick
 	}
 
 	if useAccountName {
-		if registered && newNick != accountName && newNick != "" {
+		if registered && newNick != accountName {
 			return "", errNickAccountMismatch, false
 		}
 		newNick = accountName