cleaned up Reachability

Author: Aaronontheweb

src/core/Akka.Cluster/Reachability.cs

@@ -55,7 +55,6 @@ public enum ReachabilityStatus
         public static readonly Reachability Empty =
             new Reachability(ImmutableList.Create<Record>(), ImmutableDictionary.Create<UniqueAddress, long>());
 
-        //TODO: Serialization should ignore
         private readonly Lazy<Cache> _cache;
 
         /// <summary>
@@ -80,11 +79,6 @@ public Reachability(ImmutableList<Record> records, ImmutableDictionary<UniqueAdd
         /// </summary>
         public ImmutableDictionary<UniqueAddress, long> Versions { get; }
 
-        /*
-         *  def isReachable(observer: UniqueAddress, subject: UniqueAddress): Boolean =
-            status(observer, subject) == Reachable
-         */
-
         /// <summary>
         ///     TBD
         /// </summary>
@@ -223,6 +217,9 @@ public Reachability Merge(IImmutableSet<UniqueAddress> allowed, Reachability oth
                 var rows1 = ObserverRows(observer);
                 var rows2 = other.ObserverRows(observer);
 
+                if (rows1 == null && rows2 == null)
+                    continue;
+
                 if (rows1 != null && rows2 != null)
                 {
                     var rows = observerVersion1 > observerVersion2 ? rows1 : rows2;