Preserve self-closing block notation when rewriting URLs

[adamziel]

Backports php-toolkit PR 191:

Before this PR, BlockMarkupProcessor (and BlockMarkupUrlProcessor) would lose the self-closing solidus when rewriting block attributes:

$p = new BlockMarkupUrlProcessor(
	'<!-- wp:navigation-link {"url": "https://w.org"} /-->'
);
$p->next_url();
$p->set_url('https://new-site.org', WPURL::parse('https://new-site.org'));
echo $p->__toString();
// <!-- wp:navigation-link {"url": "https://new-site.org"} -->

With this PR, the self-closing solidus is preserved:

$p = new BlockMarkupUrlProcessor(
	'<!-- wp:navigation-link {"url": "https://w.org"} /-->'
);
$p->next_url();
$p->set_url('https://new-site.org', WPURL::parse('https://new-site.org'));
echo $p->__toString();
// <!-- wp:navigation-link {"url": "https://new-site.org"} /-->

Testing instructions

CI – the php-toolkit PR has its own tests and this WordPress importer PR also comes with an additional check to ensure self-closing blocks are preserved.