fix datetime null error (#6627)

* fix datetime null error

* clean up

* enable test only on x64/x86 machines

Author: LittleLittleCloud

src/Microsoft.ML.Data/DataLoadSave/Database/DatabaseLoaderCursor.cs

@@ -307,7 +307,7 @@ private ValueGetter<byte> CreateByteGetterDelegate(ColInfo colInfo)
             private ValueGetter<DateTime> CreateDateTimeGetterDelegate(ColInfo colInfo)
             {
                 int columnIndex = GetColumnIndex(colInfo);
-                return (ref DateTime value) => value = DataReader.GetDateTime(columnIndex);
+                return (ref DateTime value) => value = DataReader.IsDBNull(columnIndex) ? default : DataReader.GetDateTime(columnIndex);
             }
 
             private ValueGetter<double> CreateDoubleGetterDelegate(ColInfo colInfo)

test/Microsoft.ML.Tests/DatabaseLoaderTests.cs

@@ -7,7 +7,9 @@
 using System.Data.SqlClient;
 using System.Data.SQLite;
 using System.IO;
+using System.Linq;
 using System.Runtime.InteropServices;
+using FluentAssertions;
 using Microsoft.ML.Data;
 using Microsoft.ML.RunTests;
 using Microsoft.ML.TestFramework;
@@ -222,6 +224,38 @@ public void IrisSdcaMaximumEntropy()
             }).PredictedLabel);
         }
 
+        [X86X64FactAttribute("The SQLite un-managed code, SQLite.interop, only supports x86/x64 architectures.")]
+        public void TestLoadDatetimeColumnWithNullValue()
+        {
+            var connectionString = "DataSource=Dummy;Mode=Memory;Version=3;Timeout=120;Cache=Shared";
+            using (var connection = new SQLiteConnection(connectionString))
+            {
+                connection.Open();
+                using (var command = new SQLiteCommand(connection))
+                {
+                    command.CommandText = """
+                        BEGIN;
+                        CREATE TABLE IF NOT EXISTS Datetime (datetime Datetime NULL);
+                        INSERT INTO Datetime VALUES (NULL);
+                        INSERT INTO Datetime VALUES ('2018-01-01 00:00:00');
+                        COMMIT;
+                        """;
+                    command.ExecuteNonQuery();
+                }
+            }
+            var mlContext = new MLContext(seed: 1);
+            var loader = mlContext.Data.CreateDatabaseLoader(new DatabaseLoader.Column("datetime", DbType.DateTime, 0));
+            var source = new DatabaseSource(SQLiteFactory.Instance, connectionString, "SELECT datetime FROM Datetime");
+            var data = loader.Load(source);
+            var datetimes = data.GetColumn<DateTime>("datetime").ToArray();
+            datetimes.Count().Should().Be(2);
+
+            // Convert null value to DateTime.MinValue, aka 0001-01-01 00:00:00
+            // This is the default behavior of TextLoader as well.
+            datetimes[0].Should().Be(DateTime.MinValue);
+            datetimes[1].Should().Be(new DateTime(2018, 1, 1, 0, 0, 0));
+        }
+
         /// <summary>
         /// Non-Windows builds do not support SqlClientFactory/MSSQL databases. Hence, an equivalent
         /// SQLite database is used on Linux and MacOS builds.