Fix nullable annotations for LFU

BitFaster.Caching/Lfu/ConcurrentLfuCore.cs

@@ -717,7 +717,7 @@ private void EvictEntries()
             EvictFromMain(candidate);
         }
 
-        private LfuNode<K, V> EvictFromWindow()
+        private LfuNode<K, V>? EvictFromWindow()
         {
             LfuNode<K, V>? first = null;
 
@@ -732,17 +732,17 @@ private LfuNode<K, V> EvictFromWindow()
                 node.Position = Position.Probation;
             }
 
-            return first!;
+            return first;
         }
 
         private ref struct EvictIterator
         {
             private readonly CmSketch<K> sketch;
-            public LfuNode<K, V> node;
+            public LfuNode<K, V>? node;
             public int freq;
 
             [MethodImpl(MethodImplOptions.AggressiveInlining)]
-            public EvictIterator(CmSketch<K> sketch, LfuNode<K, V> node)
+            public EvictIterator(CmSketch<K> sketch, LfuNode<K, V>? node)
             {
                 this.sketch = sketch;
                 this.node = node;
@@ -752,7 +752,7 @@ public EvictIterator(CmSketch<K> sketch, LfuNode<K, V> node)
             [MethodImpl(MethodImplOptions.AggressiveInlining)]
             public void Next()
             {
-                node = node.Next;
+                node = node!.Next;
 
                 if (node != null)
                 {
@@ -761,7 +761,7 @@ public void Next()
             }
         }
 
-        private void EvictFromMain(LfuNode<K, V> candidateNode)
+        private void EvictFromMain(LfuNode<K, V>? candidateNode)
         {
             var victim = new EvictIterator(this.cmSketch, this.probationLru.First); // victims are LRU position in probation
             var candidate = new EvictIterator(this.cmSketch, candidateNode);