[JENKINS-40382] GitSCMFileSystem mistakes

[jglick]

Fixes a critical bug in #455 encountered during integration testing.

Might need a backport to a different branch, I do not follow that stuff.

@reviewbybees esp. @stephenc

[ghost]

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[jglick]

Seems like there is an infrastructure problem:

[Pipeline] [split0] node
[split0] Still waiting to schedule task
[split0] All nodes of label ‘highmem’ are offline
Cancelling nested steps due to timeout

[MarkEWaite]

Thanks for the catching that @jglick . I can handle the cherry pick to the 2.6.x branch. I think I've seen that NPE in other contexts, and have been working around it by always assigning a credential (even if it is an invalid credential)