Empty env. variable prevents the tmux window from being renamed

Start in ssh_manager/tmux.py around lines 13–19 and review commit 7e1c09dc577e804f38d07654abd7e7e7ecf770ae. Reproduce the empty environment-variable case described in the issue and verify that it no longer raises SystemExit before the tmux window can be 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:

0. Re-read the README installation and configuration guide and follow all of those steps
1. 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)
2. 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
3. Verify python installation itself:
   @ python3 -V
   OR
   @ python3.x -V where's x is your python release

Checklist:

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

A detailed explanation of the issue goes here

Screenshots, screencasts, and other useful information are welcome

Since https://github.com/LoliPain/ssh_manager/commit/7e1c09dc577e804f38d07654abd7e7e7ecf770ae empty envvar causing SystemExit which prevents graceful re-rename
https://github.com/LoliPain/ssh_manager/blob/db1b9b76f6679f643c8889cc3bd97ef7cdb7307e/ssh_manager/tmux.py#L13-L19