Fix the protected function Th1::IsEmpty used in TH1Merger for the case when...

Fix the protected function Th1::IsEmpty used in TH1Merger for the case when both fTSumw=0 and fEntries=0, but in reality the bin contents are not zero.
Add in this case a check to all bin contents, including underflow/overflows
Fix also TH1::GetStats() for the case of labels histograms.  In this case when you have   fTSumw=0  and fEntries !=0 (for example after calling SetBinContent) you want to compute
the correct fTsumw and fTsumw2 values