chore: fix nullable annotation of AssertHasValue

Author: filzrev

src/BenchmarkDotNet/Characteristics/CharacteristicHelper.cs

@@ -19,7 +19,7 @@ internal static bool IsCharacteristicObjectSubclass(Type type) =>
         private static bool IsCharacteristicSubclass(Type type) =>
             type.GetTypeInfo().IsSubclassOf(typeof(Characteristic));
 
-        private static Characteristic AssertHasValue(MemberInfo member, Characteristic value)
+        private static Characteristic AssertHasValue(MemberInfo member, Characteristic? value)
         {
             if (member?.DeclaringType == null)
                 throw new NullReferenceException($"{nameof(member.DeclaringType)}");
@@ -56,12 +56,12 @@ private static IReadOnlyList<Characteristic> GetThisTypeCharacteristicsCore(
             var fieldValues = characteristicObjectType.GetTypeInfo()
                 .GetFields(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.FlattenHierarchy | BindingFlags.Static)
                 .Where(f => IsCharacteristicSubclass(f.FieldType))
-                .Select(f => AssertHasValue(f, (Characteristic)f.GetValue(null)!));
+                .Select(f => AssertHasValue(f, (Characteristic?)f.GetValue(null)));
 
             var propertyValues = characteristicObjectType.GetTypeInfo()
                 .GetProperties(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.FlattenHierarchy | BindingFlags.Static)
                 .Where(p => p.GetMethod != null && IsCharacteristicSubclass(p.PropertyType))
-                .Select(p => AssertHasValue(p, (Characteristic)p.GetValue(null)!));
+                .Select(p => AssertHasValue(p, (Characteristic?)p.GetValue(null)));
 
             // DONTTOUCH: DO NOT change the order of characteristic as it may break logic of some operations.
             return fieldValues