NIFI-15529: Fix to StatelessGroupLifecycle to ensure that we set the …

[markap14]

…desired state of Stateless Group Node before attempting to schedule; updated StandardProcessScheduler.startStatelessGroup to make it more clear if this is left out again at some point

Summary

NIFI-00000

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[bobpaulin]

Thanks for this change @markap14 . Approved non-binding