Give non null default to L0DUReportMonitor xLabelOptions, required by ROOT 6.24

Marco Cattaneorequested to merge
fixForROOT6-24 into run2-patches

Fixes the errors

Error in <TH2D::LabelsOption>:  is an invalid label placement option!

in the boole.boole-fest test with LCG_100

Edited by Marco Cattaneo

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