Is it possible to ignore the first Line Break in a preformatted Div from a shortcode?

You could try replacing the extra <br /> at the beginning of the shortcode content.

You could achieve that in many ways, but here’s an example:

if( '<br />' === substr( ltrim( $content ), 0, 6 ) )
    $content = substr( ltrim( $content ), 6 );

return '<div class="pncode">' . $content . '</div>';

where we play with the substr() and ltrim() functions.

Update:

Thanks to @userabuser for his comment.

If we want to remove all variations of br tags, from the beginning of the shortcode string, we might try to construct this kind of reg-ex replacement:

$content = preg_replace( '#^\s*(<br\s*/?>\s*)+#i', '', $content );

Example:

The shortcode content:

   <br />    <br>
<BR> <bR> 
some sample code <br />
some sample code <br />
some sample code <br />

would display as:

some sample code <br />
some sample code <br />
some sample code <br />