How to differentiate download link function if the value in the database is null?

Or Handle that in UI. If you have one more value for visibility(VisibilityFlag from backend), that's enough. It'll automatically show/hide button depends on value.

<asp:LinkButton ID="lnbResend" runat="server" Visible='<%Eval("VisibilityFlag")%>'></asp:LinkButton>

show and hide link button in grid view | The ASP.NET Forums[^]

Or Handle that in event bubbling. Check for not null in code-behind.

protected void LinkButton_Click(Object sender, CommandEventArgs e)
    {
        if (e.CommandArgument != null)//Check for not null
        {
            Response.Redirect("YourPage.aspx?IdPassed=" + e.CommandArgument.ToString());//Do what ever you want
        }
    }

https://www.hightechnology.in/how-to-redirect-to-another-page-by-click-on-linkbutton-in-gridview/[^]
You can even change the color of button or hide button based on condition

protected void LinkButton_Click(object sender, CommandEventArgs e)
{
    LinkButton btn = (LinkButton)(sender);
    string yourValue = btn.CommandArgument;
    if(btn != null)
    {
       btn.ForeColor = System.Drawing.Color.Red;    
       btn.Visible = true;    
    }
    else    {
       btn.ForeColor = System.Drawing.Color.Green;    
       btn.Visible = false;    
    }
}