[JENKINS-38527] - Prevent NullPointerException in Slave#createLauncher() and...

[JENKINS-38527] - Prevent NullPointerException in Slave#createLauncher() and add cause diagnostics (#2923)

* [JENKINS-38527] - Prevent NullPointerException in Slave#createLauncher() and add cause diagnostics

The original issue comes from the isUnix() unboxing, but we can also get into an issue later if we pass a null Channel instance to the logic.
This change adds some diagnostics which discovers potential root causes of such potential NPEs due to the race conditions with Computer reconnection

* [JENKINS-38527] - Also handle cases when Channel#isClosingOrClosed() as @stephenc suggested