Simple: don't try to do a convert:
if (txt_CheqNo.Text == "")
{
objdal.Cheq_No = 0;
}
else
{
objdal.Cheq_No = Convert.ToInt16(txt_CheqNo.Text);
}
But a better way would be:
objdal.Cheq_No = 0;
if (!string.IsNullOrWhiteSpace(txt_CheqNo.Text))
{
objdal.Cheq_No = Convert.ToInt16(txt_CheqNo.Text);
}