Fix filling of monitoring histograms with kVec/kVecUO options.

Histograms with those options were not filled at all, there was a logic bug in VecHistogramFiller1D::fill method. The fix is limited to that class only and I have added a unit test for those two options. I also noticed that HistogramDef contents is not initialized and I switched to using MockHistogramDef in couple of other unit tests where I think it may be important to have properly initialized definition. And I think HistogramDef needs a default constructor which makes instances in a known state (or per-member initializers).

Fixes ATR-20730