fix(fetch): convert null init in Response.json

[Ram-blip]

This relates to...

Fixes Response.json(data, null) so null is handled through ResponseInit conversion instead of being passed directly to response initialization.

Rationale

Response.json() takes an optional ResponseInit dictionary. Web IDL dictionary conversion allows null, treating dictionary members as absent so defaults apply.

Response.json() was skipping ResponseInit conversion when init was null, which caused raw null to reach initializeResponse() and throw when reading init.status.

This change routes null through the existing ResponseInit converter, matching the behavior already used by the Response constructor.

Changes

• Always convert init with webidl.converters.ResponseInit() in Response.json().
• Add regression coverage for Response.json(data, null) in the existing INIT_TESTS table.

Features

N/A

Bug Fixes

Fixes Response.json(data, null) throwing TypeError.

Breaking Changes and Deprecations

N/A

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.45%. Comparing base (c7c7e7f) to head (c95b060).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5540   +/-   ##
=======================================
  Coverage   93.45%   93.45%           
=======================================
  Files         110      110           
  Lines       37376    37374    -2     
=======================================
- Hits        34928    34927    -1     
+ Misses       2448     2447    -1     

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[mcollina]

lgtm