…ot user name, to exempt empty strings (lack of specific configuration) [JENKINS-72590]

Signed-off-by: Jim Klimov <jimklimov+jenkinsci@gmail.com>

…ot user name, to exempt empty strings (lack of specific configuration) if Jenkins security setting allows that [JENKINS-72590]

Signed-off-by: Jim Klimov <jimklimov+jenkinsci@gmail.com>

…row UsernameNotFoundException when we fail to impersonate() or to User.get() AND the name is blank [JENKINS-72590]

This allows e.g. IRC Bot to start and send messages ("are you talking to
me?" and build notifications) but not send commands when no Jenkins user
is attached to the IRC configuration.

Signed-off-by: Jim Klimov <jimklimov+jenkinsci@gmail.com>

…g command execution, outline this to private message sender [JENKINS-72590]

Signed-off-by: Jim Klimov <jimklimov+jenkinsci@gmail.com>