Keep empty img tags

[Strubbl]

keep empty img tags to avoid missing images, e.g. wallabag/wallabag#7700

[coveralls]

coverage: 94.897%. remained the same
when pulling cb190fc on Strubbl:patch-1
into 1281bf3 on j0k3r:master.

[jtojnar]

Thanks for the contribution.

Would you be able to include a test like the following?

graby/tests/GrabyTest.php

Lines 1000 to 1008 in 1281bf3

	public function testEmptyNodesRemoved(): void
	{
	$graby = $this->getGrabyWithMock('/fixtures/content/framablog.html');
	$res = $graby->fetchContent('https://framablog.org/2017/12/02/avancer-ensemble-vers-la-contribution/');
	// The initial treatment was encapsulating the content into the empty node
	// So we don't want to see that again
	$this->assertStringNotContainsString('<figure><p>Après un <em>icebreaker</em>', $res->getHtml());
	}

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Aside, I find it weird that #141 was made against Graby rather than Readability in the first place but whatever 🤷‍♀️

[Strubbl]

Sorry, i am not sure how to write and run a proper test here.