IMConnectionProvider.java: getAuthenticationHolder(): fix check for Bot user name…

[jimklimov]

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

Per https://issues.jenkins.io/browse/JENKINS-72590 the check was insufficient - it only excluded null values, but an empty saved config conveys an empty non-null String instead.

Testing done

Stumbled on this while dev-testing other PRs, and the temporary Jenkins controller had just a trivial configuration to log into IRC but only had an admin user defined locally. Replies from Jenkins for commands which required authorization, such as "help", failed with inability to "impersonate" and did not make it to IRC.

With this change, replies are present in a private chat and commands are honoured.

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UPDATE: Probably this is counter-productive after all: can't assign permission constraints to a non-existing user. Would change to a more reasonable exception that helps troubleshooting instead.

[jimklimov]

With the mood change to throw an exception for the empty user-name config in the later commit, the IRC client in fact is precluded from starting (which may be counter-productive for sites that only want to post notifications):

2024-01-22 23:13:37.779+0000 [id=82]    INFO    h.p.i.IMConnectionProvider$ConnectorRunnable#run: Trying to reconnect
2024-01-22 23:13:37.793+0000 [id=82]    SEVERE  h.i.i.InstallUncaughtExceptionHandler$DefaultUncaughtExceptionHandler#uncaughtException: A thread (IM-Reconnector-Thread/82) died unexpectedly due to an uncaught exception. This may leave your
server corrupted and usually indicates a software bug.
org.acegisecurity.userdetails.UsernameNotFoundException: No local Jenkins user name is configured for instant messaging to act as
        at hudson.plugins.im.IMConnectionProvider.getAuthenticationHolder(IMConnectionProvider.java:127)
        at hudson.plugins.ircbot.v2.IRCConnectionProvider.createConnection(IRCConnectionProvider.java:40)
        at hudson.plugins.im.IMConnectionProvider.create(IMConnectionProvider.java:64)
        at hudson.plugins.im.IMConnectionProvider$ConnectorRunnable.run(IMConnectionProvider.java:193)
        at java.base/java.lang.Thread.run(Thread.java:833)

[jimklimov]

With the latest change, this gets well-balanced: IRC bot can start and send notifications, but can not run commands, when not properly configured with a Jenkins user.