How to write an empty value for points on Teams?

Question

I'm developing Windows Form app using Graph API. I have Excel file with data from Teams. Excel columns are: Display name, Feedback, Points,Submission id, OutcomeFeedback id and OutcomePoints id. I want to update the value of points in the Excel file and write that value on Teams. The problem is that when I leave the field for points empty, I get an error. Does anyone know how to solve this? Here is the code:

for (int i = 2; i <= rowCount; i++)
{
    string cellValue1 = Convert.ToString(excelWorksheet.Cells[i, 2].Value); //feedback
    string cellValue2 = Convert.ToString(excelWorksheet.Cells[i, 3].Value); //points
    string cellValue3 = Convert.ToString(excelWorksheet.Cells[i, 4].Value); //submission id
    string cellValue4 = Convert.ToString(excelWorksheet.Cells[i, 5].Value); //outcome feedback id
    string cellValue5 = Convert.ToString(excelWorksheet.Cells[i, 6].Value); //outcome points

    if(cellValue1 == null)
    {
        await GraphHelper.UpdateFeedback("", this.team_id, assignment_id, cellValue3, cellValue4);
    }
    else
    {
        await GraphHelper.UpdateFeedback(cellValue1, this.team_id, assignment_id, cellValue3, cellValue4);
    }

    if(cellValue2 == null)
    {
        await GraphHelper.UpdatePoints("", this.team_id, assignment_id, cellValue3, cellValue5);
    }
    else if(Int32.Parse(cellValue2) <= 100 && Int32.Parse(cellValue2) >= 0)
    {
        await GraphHelper.UpdatePoints(cellValue2, this.team_id, assignment_id, cellValue3, cellValue5);
    }
}

public static async Task UpdatePoints(string points, string teamId, string assignmentId, string submissionId, string outcomeId)
{
    graphClient = GetGraphClient(token);

    var educationOutcomePoint = new EducationPointsOutcome
    {
        Points = new EducationAssignmentPointsGrade
        {
            Points = Int32.Parse(points)
        }
    };

    await GraphHelper.graphClient.Education.Classes[teamId].Assignments[assignmentId].Submissions[submissionId].Outcomes[outcomeId]
         .Request()
        .UpdateAsync(educationOutcomePoint);
}