Fix TryGetEntityName for custom entity names

Author: maca88

src/NHibernate/Async/Persister/Entity/IEntityPersister.cs

@@ -20,7 +20,6 @@
 using System.Collections.Generic;
 using NHibernate.Intercept;
 using NHibernate.Util;
-using System.Linq;
 
 namespace NHibernate.Persister.Entity
 {

src/NHibernate/Linq/Visitors/NullableExpressionDetector.cs

@@ -163,8 +163,8 @@ private bool IsNullable(MemberExpression memberExpression, BinaryExpression equa
 			}
 
 			// We have to check the member mapping to determine if is nullable
-			var entityName = ExpressionsHelper.TryGetEntityName(_sessionFactory, memberExpression, out var memberPath);
-			if (entityName == null)
+			var entityName = ExpressionsHelper.TryGetEntityName(_sessionFactory, memberExpression, out var memberPath, out _);
+			if (entityName == null || memberPath == null)
 			{
 				return true; // Not mapped
 			}