Log: Ignore null connections (fast connect/close)

[ggrandes]

The typical ELB health-checks:

2023-05-15 18:27:48,047 INFO org.javastack.sftpserver.ServiceLogger session created(@10.x.x.xx:37118)
2023-05-15 18:27:48,047 INFO org.javastack.sftpserver.ServiceLogger session closed(@10.x.x.xx:37118)
2023-05-15 18:27:48,063 INFO org.javastack.sftpserver.ServiceLogger session created(@10.x.x.xx:37114)
2023-05-15 18:27:48,063 INFO org.javastack.sftpserver.ServiceLogger session closed(@10.x.x.xx:37114)
2023-05-15 18:27:48,548 INFO org.javastack.sftpserver.ServiceLogger session created(@10.x.x.xx:9088)
2023-05-15 18:27:48,548 INFO org.javastack.sftpserver.ServiceLogger session closed(@10.x.x.xx:9088)
...and many many more