Image->iptc('attribute') returns Image instead of null

[Swanty]

The problem is here:
https://github.com/Intervention/image/blob/2.3.8/src/Intervention/Image/Image.php#L107

IPTC tests don't catch it since, they're using IptcCommand directly:
https://github.com/Intervention/image/blob/2.3.8/tests/IptcCommandTest.php

In normal usage developers would use Image->iptc('attribute') as it is described in the documentation here:
http://image.intervention.io/api/iptc

To reproduce this, just try to get an iptc attribute that is not set inside the image.
IptcCommand will set output to null, but Image __call will return Image object

[olivervogel]

Can't reproduce the described bug.

1. The IptcCommand sets the output to false if the attribute is not found.
2. __call will check if command hasOutput, in this case false is output, so hasOutput will return true.
3. $command->getOutput() is returned, which will resolve to null in this case.