Update LineSeries.cs
Update LineSeries.cs

Prevent null reference with MVVM binding values, example:
<OxyPlot:LineSeries ItemsSource="{Binding MyIEnumerableValueCollection}"  />
svendu/oxyplot,freudenthal/oxyplot,lynxkor/oxyplot,freudenthal/oxyplot,svendu/oxyplot,as-zhuravlev/oxyplot_wpf_fork,Sbosanquet/oxyplot,freudenthal/oxyplot,NilesDavis/oxyplot,bbqchickenrobot/oxyplot,TheAlmightyBob/oxyplot,Jofta/oxyplot,lynxkor/oxyplot,HermanEldering/oxyplot,objorke/oxyplot,br111an/oxyplot,Rustemt/oxyplot,Sbosanquet/oxyplot,as-zhuravlev/oxyplot_wpf_fork,jeremyiverson/oxyplot,shoelzer/oxyplot,Isolocis/oxyplot,Sbosanquet/oxyplot,shoelzer/oxyplot,bbqchickenrobot/oxyplot,jeremyiverson/oxyplot,Kaplas80/oxyplot,zur003/oxyplot,sevoku/oxyplot,jeremyiverson/oxyplot,Rustemt/oxyplot,DotNetDoctor/oxyplot,TheAlmightyBob/oxyplot,H2ONaCl/oxyplot,shoelzer/oxyplot,GeertvanHorrik/oxyplot,DotNetDoctor/oxyplot,TheAlmightyBob/oxyplot,olegtarasov/oxyplot,objorke/oxyplot,olegtarasov/oxyplot,Mitch-Connor/oxyplot,ze-pequeno/oxyplot,Rustemt/oxyplot,Mitch-Connor/oxyplot,HermanEldering/oxyplot,oxyplot/oxyplot,svendu/oxyplot,br111an/oxyplot,GeertvanHorrik/oxyplot,BRER-TECH/oxyplot,GeertvanHorrik/oxyplot,as-zhuravlev/oxyplot_wpf_fork,BRER-TECH/oxyplot,H2ONaCl/oxyplot,H2ONaCl/oxyplot,bbqchickenrobot/oxyplot,Jonarw/oxyplot,mattleibow/oxyplot,mattleibow/oxyplot,Kaplas80/oxyplot,lynxkor/oxyplot,sevoku/oxyplot,objorke/oxyplot,Mitch-Connor/oxyplot,br111an/oxyplot