It seems that this method can return a zero-length string if faced with large-ish data. No exceptions are thrown.
<?php
$image = new Imagick();
if (!$image->newImage(1000,1,'white')) throw new Exception();
if (!$image->scaleImage(0,200)) throw new Exception();
print "Image size: {$image->getImageWidth()},{$image->getImageHeight()}\n";
if (!$image->setImageFormat("jpeg")) throw new Exception();
$a = $image->getImageBlob();
print "Rendered to ".strlen($a)." bytes\n";
?>
Restrict your output image size, or ensure that the blob you get back isn't empty. (Note that IM seems to be doing the work, it delays for some time. But there's no indication of any error anywhere.)