rrepair, recon: fix wrong gb_sets:is_empty/1 call when it should be gb_trees:is_empty/1
rrepair, recon: fix wrong gb_sets:is_empty/1 call when it should be gb_trees:is_empty/1

(found by dialyzer)
digshock/scalaris,seyyed/scalaris,digshock/scalaris,seyyed/scalaris,seyyed/scalaris,seyyed/scalaris,tectronics/scalaris,digshock/scalaris,tectronics/scalaris,digshock/scalaris,tectronics/scalaris,tectronics/scalaris,tectronics/scalaris,digshock/scalaris,seyyed/scalaris,tectronics/scalaris,seyyed/scalaris,tectronics/scalaris,digshock/scalaris,digshock/scalaris,seyyed/scalaris