fix: Link target attribute parsing to match serialization

[dkesberg]

Description / Problem

When parsing HTML links without a target attribute, the parser returns null from getAttrs, leaving target unset in the mark's attrs.
During rendering, renderHTML would then apply the default target="_blank" from HTMLAttributes.
In the current tests for serialization the test link mark can disable target (tests/DOMSerializer/Marks/LinkTest.php:250) tests that a mark with target: null does not output the _blank default in rendered HTML.
To be consistent the parser should also initialize the document object with target: null.

Solution

getAttrs now explicitly returns target: null when no target attribute is present on the parsed element.
This mirrors the existing serializer behavior: a mark with target: null does not output target="_blank"

Changes

• src/Marks/Link.php: return ['target' => null] from getAttrs when no target is present
• tests/DOMParser/Marks/LinkTest.php: update existing tests to reflect target: null in parsed output