Handle null logger name for acceptable check

[cholmes-axoni]

The acceptable check doesn't handle null logger names as the mkEntry does. This prevents it from failing for that null value.

This is an addition to a previous change. #1990

Let me know if this isn't the desired behavior and if records with null logger names should either always be accepted or never accepted.

[error]   NullPointerException
[error] 
[error]   StringOps.scala:836               scala.collection.StringOps$#split$extension
[error]   LogConfigServiceImpl.scala:21     izumi.logstage.api.routing.LogConfigServiceImpl#threshold
[error]   ConfigurableLogRouter.scala:38    izumi.logstage.api.routing.ConfigurableLogRouter#acceptable
[error]   LogstageJulLogger.scala:22        izumi.logstage.adapter.jul.LogstageJulLogger#publish
[error]   Logger.java:979                   java.util.logging.Logger#log

[neko-kai]

Thank you very much! 🙏