histogram: fix merge of single valued histogram
histogram: fix merge of single valued histogram

tectronics/scalaris,tectronics/scalaris,tectronics/scalaris,tectronics/scalaris,digshock/scalaris,seyyed/scalaris,digshock/scalaris,digshock/scalaris,digshock/scalaris,seyyed/scalaris,seyyed/scalaris,seyyed/scalaris,seyyed/scalaris,tectronics/scalaris,digshock/scalaris,seyyed/scalaris,digshock/scalaris,tectronics/scalaris,tectronics/scalaris,seyyed/scalaris,digshock/scalaris