Search first empty space in the database

I have a small application in C #, which creates the auto-supplier code. I would like to know if anyone knows any way the system can look for an empty code, eg:

  

1 - Manoel

     

2 - Narcissus

     

4 - Mustela

The next code he should pull would be 3 , but he always goes to the last record. Here is the code:

private void btnGravar_Click(object sender, EventArgs e)
{
    string Id = txtID.Text;
    string Nome = txtNome.Text;
    string CNPJ = txtCNPJ.Text;
    if (txtNome.Text == "" || txtCNPJ.Text=="" )
    {
        MessageBox.Show("Favor informar os dados para cadastro!", "Alerta",
        MessageBoxButtons.OK,
        MessageBoxIcon.Warning);
        return;
    }

    try
    {
        CONEXAO.Open();
        comando = new MySqlCommand("INSERT INTO fornecedores (ID, Nome, CNPJ)" + "VALUES ('"
            + Id + "','" + Nome + "','" + CNPJ + "')", CONEXAO);
        comando.Parameters.AddWithValue(Id, txtID.Text);
        comando.Parameters.AddWithValue(Nome, txtNome.Text);
        comando.Parameters.AddWithValue(CNPJ, txtCNPJ.Text);
        comando.ExecuteNonQuery();
        // Registra os fornecedores no Data Grid.
        registraFornecedor(txtID.Text, txtNome.Text, Convert.ToString(txtCNPJ.Text));
        ClearControls();
    }
    catch
    {
        MessageBox.Show("A conexão com o banco de dados falhou");
    }
    finally
    {
        CONEXAO.Close();
    }
}