Throw exception instead of returning NULL

The signature of the function tells us that getImageBlob()
always return string or throws Exception.

In this case, the returning value is NULL as retval is unset.

imagick_class.c

@@ -8409,7 +8409,8 @@ PHP_METHOD(Imagick, getImageBlob)
 
 	image_contents = MagickGetImageBlob(intern->magick_wand, &image_size);
 	if (!image_contents) {
-		return;
+		php_imagick_throw_exception(IMAGICK_CLASS, "Failed to get the image contents (empty or invalid image?)" TSRMLS_CC);
+		RETURN_THROWS();
 	}
 
 	IM_ZVAL_STRINGL(return_value, (char *)image_contents, image_size);

tests/329_imagick_getImageBlob_empty.phpt

@@ -10,14 +10,14 @@ $imagick = new Imagick();
 try {
     $imagick->newPseudoImage(200, 200, "xc:red");
 	$result = $imagick->getImageBlob();
-	echo "Imagick failed to throw exception";
+	echo "Imagick failed to throw exception" . PHP_EOL;
 } catch (ImagickException $e) {
-	echo "ImagickException: " . $e->getMessage();
+	echo "ImagickException: " . $e->getMessage() . PHP_EOL;
 }
 
 echo "Fin.\n";
 
 ?>
 --EXPECTF--
-ImagickException: Some words here.
+ImagickException: Failed to get the image contents (empty or invalid image?)
 Fin.