Fix nullable values handlings and add tests

Author: fabien.menager

src/PhenX.EntityFrameworkCore.BulkInsert/Metadata/ColumnMetadata.cs

@@ -24,7 +24,7 @@ internal sealed class ColumnMetadata(IProperty property,  SqlDialectBuilder dial
 
     public bool IsGenerated { get; } = property.ValueGenerated != ValueGenerated.Never;
 
-    public object? GetValue(object entity, BulkInsertOptions options)
+    public object GetValue(object entity, BulkInsertOptions options)
     {
         var result = _getter(entity);
 
@@ -40,7 +40,7 @@ internal sealed class ColumnMetadata(IProperty property,  SqlDialectBuilder dial
             }
         }
 
-        return result;
+        return result ?? DBNull.Value;
     }
 
     private static Func<object, object?> BuildGetter(IProperty property)

tests/PhenX.EntityFrameworkCore.BulkInsert.Tests/DbContext/TestEntity.cs

@@ -22,6 +22,13 @@ public class TestEntity : TestEntityBase
     [Column("the_identifier")]
     public Guid Identifier { get; set; }
 
+    [Column("nullable_identifier")]
+    public Guid? NullableIdentifier { get; set; }
+
+    public DateTime Created { get; set; }
+
+    public DateTime? Modified { get; set; }
+
     [Column("string_enum_value")]
     public StringEnum StringEnumValue { get; set; }