Empty env. variable prevents the tmux window from being renamed

Definitely a `ssh_manager` issue

There's a non-zero chance of your issue to pend isn't really related to ssh_manager, so please make sure that:

Re-read the README installation and configuration guide and follow all of those steps
Verify that $servername_user environment that should contain the password for the remote machine exists in your current shell session: @ echo $servername_user (Where servername and user are matching the entry in ssh_manager storage)
Verify sshpass -p direct connection to remote: @ sshpass -p $servername_user ssh user@remoteip NOTE that user in $servername_user should be exactly the same as user in ssh user@remoteip
Verify python installation itself: @ python3 -V OR @ python3.x -V where's x is your python release

Checklist:

[x] My issue is not related to connection problems, or I'm aware that my ssh_manager storage and shell environment is fine
- [x] Direct connection using sshpass -p is performed, but connection through ssh_manager seems inappropriate
[x] Broken functionality is declared and documented in the README project description