Description of problem:
Setting REX with sudo with effective user password (I assume with REX user password it will be the same), when the password contains some wide characters, REX job fails with output

output: 'Error initializing command: NoMethodError - undefined method `empty?''
for nil:NilClass'

and relevant backtrace in proxy.log (on the Sat/Caps that was attempting to ssh to the client system).

Version-Release number of selected component (if applicable):
6.12 + (any running preflight checks)

How reproducible:
100%

Steps to Reproduce:
1. On a client, configure some user with password containing a wide character (I used password čislo where the č did it)
2. Set the user as a sudoer requiring password
3. Invoke a REX job via ssh with:
effective_user_method: sudo
ssh_user: the-sudo-user
Effective user password : the password from 1.

Actual results:
The REX job fails with below backtrace in /var/log/foreman-proxy/proxy.log:

2024-01-15T11:21:42 [E] error while initializing command NoMethodError undefined method `empty?' for nil:NilClass:
/usr/share/gems/gems/smart_proxy_dynflow-0.8.2/lib/smart_proxy_dynflow/io_buffer.rb:87:in `write_available!'
/usr/share/gems/gems/smart_proxy_dynflow-0.8.2/lib/smart_proxy_dynflow/process_manager.rb:130:in `each'
/usr/share/gems/gems/smart_proxy_dynflow-0.8.2/lib/smart_proxy_dynflow/process_manager.rb:130:in `process'
/usr/share/gems/gems/smart_proxy_dynflow-0.8.2/lib/smart_proxy_dynflow/process_manager.rb:65:in `run!'
/usr/share/gems/gems/smart_proxy_remote_execution_ssh-0.7.3/lib/smart_proxy_remote_execution_ssh/runners/script_runner.rb:327:in `run_sync'
/usr/share/gems/gems/smart_proxy_remote_execution_ssh-0.7.3/lib/smart_proxy_remote_execution_ssh/runners/script_runner.rb:404:in `ensure_remote_command'
/usr/share/gems/gems/smart_proxy_remote_execution_ssh-0.7.3/lib/smart_proxy_remote_execution_ssh/runners/script_runner.rb:167:in `preflight_checks'
/usr/share/gems/gems/smart_proxy_remote_execution_ssh-0.7.3/lib/smart_proxy_remote_execution_ssh/runners/script_runner.rb:147:in `start'
/usr/share/gems/gems/smart_proxy_dynflow-0.8.2/lib/smart_proxy_dynflow/runner/dispatcher.rb:32:in `start_runner'
/usr/share/gems/gems/dynflow-1.6.10/lib/dynflow/actor.rb:13:in `on_message'
..

Expected results:
No such error.

Additional info:
ruby IO sending the wide character reports back it sent multiple chars instead just the one, causing @buffer underflow in io_buffer.rb .

aruzicka++ for the idea / collaboration