needed a null check in parsedown.php

[unmodify]

Love the project, all the best:

$line was null, which hurt my php.
So, my fix:
line num #144 in Parsedown.php:
foreach ($parts as $part)
{
$lineLength=0;
if ($line)
{
$lineLength = mb_strlen($line, 'utf-8');
}
$shortage = 4 - $lineLength % 4;

$line .= str_repeat(' ', $shortage);
$line .= $part;

}