FakeClosurePlot::init: prepend _ to tag
FakeClosurePlot::plotAll : bugfix label (was missing channel)
bugfix FakeClosurePlot::buildHists, sys and SM histos swapped

Details:
this bug was introduced when re-ordering the instructions to get the
histos. Because the histos are being re-used (with SetName), checking
the validity of their pointers all in one go is not possible.
FakeClosurePlot::plotAll: skip empty plots
add plotFakeClosure step