[3.9.x][MNG-7716] ConcurrencyDependencyGraph deadlock if no root is selected

[laeubi]

If ConcurrencyDependencyGraph#getRootSchedulableBuilds returns an empty list then MultiThreadedBuilder is locked forever as it never gets a build result (because nothing is scheduled).

This changes the method, that in such case just the first project is returned, this might not give the best performance, but ensures that there is at least one build scheduled and the build-loop can proceed.

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