fix regression/merge

(cherry picked from commit 806d6bf)

Author: MaceWindu

Source/LinqToDB.EntityFrameworkCore/EFCoreMetadataReader.cs

@@ -141,10 +141,14 @@ static IEnumerable<InheritanceMappingAttribute> GetMappingAttributesRecursive(IE
 
 			List<InheritanceMappingAttribute> ProcessEntityType(IEntityType et)
 			{
-				mappings.Add(new()
+				if (!et.ClrType.IsAbstract)
 				{
-					Type = et.ClrType, Code = entityType.GetDiscriminatorValue()
-				});
+					mappings.Add(new()
+					{
+						Type = et.ClrType,
+						Code = entityType.GetDiscriminatorValue()
+					});
+				}
 
 				if (et.BaseType == null)
 					return mappings;

Tests/LinqToDB.EntityFrameworkCore.BaseTests/ForMappingTestsBase.cs

@@ -168,13 +168,14 @@ public virtual async Task TestInheritance()
 			using var context = CreateContext();
 			using var connection = context.CreateLinqToDBConnection();
 
+			context.WithInheritance.AddRange(new List<WithInheritanceA>() { new() { } });
 			context.WithInheritance.AddRange(new List<WithInheritanceA1>() { new() { }, new() { } });
 			context.WithInheritance.AddRange(new List<WithInheritanceA2>() { new() { }, new() { } });
 			await context.SaveChangesAsync();
 
 			var result = context.GetTable<WithInheritanceA>().ToList();
 
-			result.OfType<WithInheritance>().Should().HaveCount(4);
+			result.OfType<WithInheritance>().Should().HaveCount(5);
 			result.OfType<WithInheritanceA1>().Should().HaveCount(2);
 			result.OfType<WithInheritanceA2>().Should().HaveCount(2);
 		}