fix(engine): return null from IsHTMLDDA [[Call]] per Annex B §B.3.6.1

[xcb3d]

This Pull Request fixes the return value of IsHTMLDDA's [[Call]] internal method.

According to ECMAScript Annex B §B.3.6.1 and test262's INTERPRETING.md, objects with the [[IsHTMLDDA]] internal slot (such as $262.IsHTMLDDA and document.all) must return null when called with no arguments or with an empty string.

It changes the following:

• Change is_html_dda_call in core/engine/src/builtins/is_html_dda.rs to return JsValue::null() instead of JsValue::undefined().
• Update doc comments to cite Annex B §B.3.6.1.
• Resolves all 6 failing test262 tests in test/annexB/built-ins/String/prototype/ (replace, replaceAll, match, matchAll, split, search custom-*-emulates-undefined.js), bringing the suite to 100% conformance (111/111 passed).

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	53,578	53,578	0
Passed	51,426	51,432	+6
Ignored	1,648	1,648	0
Failed	504	498	-6
Panics	0	0	0
Conformance	95.98%	95.99%	+0.01%

Fixed tests (6):

test/annexB/built-ins/String/prototype/split/custom-splitter-emulates-undefined.js (previously Failed)
test/annexB/built-ins/String/prototype/replaceAll/custom-replacer-emulates-undefined.js (previously Failed)
test/annexB/built-ins/String/prototype/search/custom-searcher-emulates-undefined.js (previously Failed)
test/annexB/built-ins/String/prototype/replace/custom-replacer-emulates-undefined.js (previously Failed)
test/annexB/built-ins/String/prototype/matchAll/custom-matcher-emulates-undefined.js (previously Failed)
test/annexB/built-ins/String/prototype/match/custom-matcher-emulates-undefined.js (previously Failed)

Tested main commit: 4b61183443403997a98da829dae287ce10358e1c
Tested PR commit: ac1a379d483adfbf63188b17ad46b2d7a17a6b7c
Compare commits: 4b61183...ac1a379

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.89%. Comparing base (6ddc2b4) to head (ac1a379).
⚠️ Report is 1051 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5511       +/-   ##
===========================================
+ Coverage   47.24%   62.89%   +15.65%     
===========================================
  Files         476      536       +60     
  Lines       46892    60286    +13394     
===========================================
+ Hits        22154    37918    +15764     
+ Misses      24738    22368     -2370     

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[jedel1043]

Good catch!