DataFrame DateTime column Min and Max calculation doesn't support NULL

[asmirnov82]

Similar to #6659, but for Max(), Min() and Sum().

New Unit tests for Max and Min:

[Fact]
public void TestDateTimeComputations_MaxMin_WithNulls()
{
    var dateTimeColumn = new DateTimeDataFrameColumn("DateTime", new DateTime?[]
        {
            null,
            new DateTime(2022, 1, 1),
            new DateTime(2020, 1, 1),
            new DateTime(2023, 1, 1),
            new DateTime(2021, 1, 1),
            null
        });

         Assert.Equal(new DateTime(2020, 1, 1), dateTimeColumn.Min());
         Assert.Equal(new DateTime(2023, 1, 1), dateTimeColumn.Max());
}

[Fact]
public void TestIntComputations_MaxMin_WithNulls()
{
    var column = new Int32DataFrameColumn("Int", new int?[]
        {
            null,
            2,
            1,
            4,
            3,
            null
        });

    Assert.Equal(1, column.Min());
    Assert.Equal(4, column.Max());
}

fail with Messages: 

Assert.Equal() Failure
Expected: 1
Actual: 0

and

Assert.Equal() Failure
Expected: 2020-01-01T00:00:00.0000000
Actual: 0001-01-01T00:00:00.0000000

Unit Test:

[Fact]
public void TestComputations_MaxMin_OnEmptyColumn()
{
    var column = new Int32DataFrameColumn("Int");

    Assert.Null(column.Min());
    Assert.Null(column.Max());
}

crashes with

System.ArgumentOutOfRangeException : Index was out of range. Must be non-negative and less than the size of the collection. (Parameter 'index')

New Unit tests for Sum:

[Fact]
public void TestIntSum_OnColumnWithNullsOnly()
{
    var column = new Int32DataFrameColumn("Int", new int?[] { null, null });
    Assert.Null(column.Sum());
}

[Fact]
public void TestIntSum_OnEmptyColumn()
{
    var column = new Int32DataFrameColumn("Int");
    Assert.Null(column.Sum());
}

crashes with

_System.ArgumentOutOfRangeException : Index was out of range. Must be non-negative and less than the size of the collection. (Parameter 'index')

[JakeRadMSFT]

Merged into Generic Math branch!