Fixed null deprecation in Zend/Pdf/Resource/Font/Simple

[kiatng]

I tried to fix the iconv() null deprecation on parameter #3 in the sources that use the PDF lib, but that proved to be very difficult to pin. Even numerous fixes in drawText() (maybe it's not from it) the errors keep popping up.

[develart-projects]

I think we can replace

if ($string === null || $string === '') { return 0; }

with

if ( empty($string) ) { return 0; }

..to make things more readable :)

[sreichel]

I think we can replace

No. Empty is bad.

$string = '0';
var_dump(($string === null || $string === '') === (empty($string)));

[develart-projects]

OK, got the point, thanks.