Fix notification mail template header-body splitting
Since file() doesn't remove "\n" from the lines read the condition
$lines[$l] == '' never evaluated to true.
So $mailHeaders would always contain every line from the template while
$mailBody contained none of them.
Together with the bugfix of PHP#68776 [1] disallowing multiple "\n"'s & preceding "\n" in mail()'s $additionalHeaders arg this caused mails not being sent because $mailHeaders would include at least one "\n\n" which was meant to separate the mail headers from the body.