The function handle_get_attribute should act differently when the
attribute is a boolean attribute.
The full list of attributes can be found in [1]. All attributes marked
as "Boolean attribute" in the "Value" column are boolean attributes.
Note that "hidden" is effectively treated as a boolean attribute,
according to WPT test "test_global_boolean_attributes" in
webdriver/tests/classic/get_element_attribute/get.py
[1] https://html.spec.whatwg.org/multipage/#attributes-3
Testing: Updated WPT test expectation
Fixes: #38353
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Signed-off-by: Kingsley Yung <kingsley@kkoyung.dev>