arg null checks on public methods
arg null checks on public methods

adding sanity checks that prevent the following weird null ref error:
System.NullReferenceException: Object reference not set to an instance
of an object.
at N2.Persistence.VersionManager.SaveVersion(ContentItem item) in
C:\Users\Victor\Documents\GitHub\n2cms\src\Framework\N2\Persistence\VersionManager.cs:line
64
at N2.Persistence.VersionManager.ReplaceVersion(ContentItem currentItem,
ContentItem replacementItem, Boolean storeCurrentVersion) in
C:\Users\Victor\Documents\GitHub\n2cms\src\Framework\N2\Persistence\VersionManager.cs:line
95
at N2.Edit.AutoPublish.PublishScheduledAction.Execute() in
C:\Users\Victor\Documents\GitHub\n2cms\src\Mvc\MvcTemplates\N2\Content\AutoPublish\PublishScheduledAction.cs:line
40
at
N2.Plugin.Scheduling.Scheduler.<>c__DisplayClass10.<ExecutActions>b__e()
in
C:\Users\Victor\Documents\GitHub\n2cms\src\Framework\N2\Plugin\Scheduling\Scheduler.cs:line
108

n2cms/n2cms,n2cms/n2cms,nicklv/n2cms,DejanMilicic/n2cms,VoidPointerAB/n2cms,n2cms/n2cms,EzyWebwerkstaden/n2cms,nimore/n2cms,DejanMilicic/n2cms,bussemac/n2cms,bussemac/n2cms,EzyWebwerkstaden/n2cms,VoidPointerAB/n2cms,nicklv/n2cms,SntsDev/n2cms,bussemac/n2cms,VoidPointerAB/n2cms,nimore/n2cms,nimore/n2cms,EzyWebwerkstaden/n2cms,SntsDev/n2cms,nicklv/n2cms,n2cms/n2cms,SntsDev/n2cms,EzyWebwerkstaden/n2cms,bussemac/n2cms,VoidPointerAB/n2cms,EzyWebwerkstaden/n2cms,nicklv/n2cms,nicklv/n2cms,bussemac/n2cms,DejanMilicic/n2cms,nimore/n2cms,DejanMilicic/n2cms,SntsDev/n2cms