tracking/list: Show "Unknown" if altitude is unknown
tracking/list: Show "Unknown" if altitude is unknown

Turbo87/skylines,RBE-Avionik/skylines,shadowoneau/skylines,RBE-Avionik/skylines,Turbo87/skylines,TobiasLohner/SkyLines,Turbo87/skylines,skylines-project/skylines,skylines-project/skylines,shadowoneau/skylines,kerel-fs/skylines,RBE-Avionik/skylines,snip/skylines,shadowoneau/skylines,TobiasLohner/SkyLines,shadowoneau/skylines,Harry-R/skylines,Harry-R/skylines,skylines-project/skylines,Harry-R/skylines,TobiasLohner/SkyLines,snip/skylines,kerel-fs/skylines,kerel-fs/skylines,snip/skylines,RBE-Avionik/skylines,Harry-R/skylines,Turbo87/skylines,skylines-project/skylines