Skip the test if Imagick is misconfigured

[nickvergessen]

cat /etc/ImageMagick-6/policy.xml

<?xml version="1.0" encoding="UTF-8"?>
…
<policymap>
  …
  <policy domain="coder" rights="none" pattern="EPS" />
  <policy domain="coder" rights="none" pattern="PDF" />
  <policy domain="coder" rights="none" pattern="XPS" />
</policymap>

EPS files are excluded by the new default imagick policy. So our test that uses them throws:

attempt to perform an operation not allowed by the security policy `PS' @ error/constitute.c/IsCoderAuthorized/408

And continues with null in:

server/lib/private/Preview/Bitmap.php

Lines 52 to 57 in caff102

	\OC::$server->getLogger()->logException($e, [
	'message' => 'File: ' . $file->getPath() . ' Imagick says:',
	'level' => ILogger::ERROR,
	'app' => 'core',
	]);
	return null;