Pass parameter to SqlCommand if Not NULL or Empty using C# in ASP.Net

I want to create a search page having 4 dropdown lists control. These parameter must be optional. i.e. result displayed should be according to the number of parameter selected outof 4.

stored procedure is working fine

@deptName nvarchar(100) = null,
@desiName nvarchar(100) = null
as
begin
	select ID,SystemType as [Product],ZoneName as Zone,DeptName as [Department],DesiName as [Designation],
	IssuedTo as [Issued To],ModelName as [Model],IssuedOn as [Issued On],ItemMake [Make],ProCompany as Company,
	UpsBattery as [UPS Battery],WinVersion as [Win Version],SysLicense as [System License],SerialKey as [Serial Key],
	NumberStatus as [Status],SysDescription as [Description] from tblSystemRegistration 
	where SystemType=@productType and (ZoneName=@zoneName or @zoneName is null) and (DeptName=@deptName or @deptName is null) 
	and (DesiName=@desiName or @desiName is null)
end

business logic

public DataTable GetSystemReport(string styp, string zon, string dept, string desi)
    {
        SqlConnection con = new SqlConnection();
        con.ConnectionString = CS;
        SqlDataAdapter adp = new SqlDataAdapter("spGetSystemInventory", con);
        adp.SelectCommand.CommandType = CommandType.StoredProcedure;
        adp.SelectCommand.Parameters.AddWithValue("@productType",styp);
        adp.SelectCommand.Parameters.AddWithValue("@zoneName", zon);
        adp.SelectCommand.Parameters.AddWithValue("@deptName", dept);
        adp.SelectCommand.Parameters.AddWithValue("@desiName", desi);
        DataTable dtp = new DataTable();
        adp.Fill(dtp);
        return dtp;
    }

.aspx.cs page

protected void btnSearchInfo_Click(object sender, EventArgs e)
    {
        strtype = ddlProductSubtype.SelectedItem.ToString() != null ? ddlProductSubtype.SelectedItem.ToString() : "";
        strZon = ddlZone.SelectedItem.ToString() != null ? ddlZone.SelectedItem.ToString() : "";
        strDept = ddlDept.SelectedItem.ToString() != null ? ddlDept.SelectedItem.ToString() : "";
        strDesi = ddlDesi.SelectedItem.ToString() != null ? ddlDesi.SelectedItem.ToString() : "";

        gvSearchRecord.DataSource = GetSystemReport(strtype, strZon, strDept, strDesi);
        gvSearchRecord.DataBind();
        
    }

what is wrong in this code ?