Lucene.Net.Support.ConcurrentSet: Fixed nullable reference type support warnings
Lucene.Net.Support.ConcurrentSet: Fixed nullable reference type support warnings

jeme/lucenenet,apache/lucenenet,NightOwl888/lucenenet,jeme/lucenenet,apache/lucenenet,jeme/lucenenet,apache/lucenenet,apache/lucenenet,NightOwl888/lucenenet,NightOwl888/lucenenet,jeme/lucenenet,NightOwl888/lucenenet