If metabox fields has content display content

by

In your php file, replace:

<tr >
<td>Brennwert</td >
<td >'. $Product_Brennwert . '</td>
</tr >

with:

';
$Product_Brennwert = get_post_meta(get_the_ID(), "Product_Brennwert", true);
if(!empty($Product_Brennwert)){
    $grid_details .= "
        <tr>
        <td>Brennwert</td >
        <td>".$Product_Brennwert."</td>
        </tr>
    ";
}
$grid_details .= '