ffi: remove dead null check in callback arguments

[trivikr]

InvokeCallback tested args[i] == nullptr and mapped the argument to JS null. args is libffi's avalue array, and libffi always points each slot at its own storage for the corresponding argument, so the slot pointers are never null and the branch never ran.

The check also read as a guarantee the code does not provide: a NULL pointer argument surfaces as the BigInt 0n, because ToJSArgument converts ffi_type_pointer values with BigInt::NewFromUnsigned. Drop the branch rather than reimplementing it in ToJSArgument, which would change behavior by making pointer parameters arrive as either a BigInt or null.

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Assisted-by: claude:opus-5

[nodejs-github-bot]

Review requested:

• @nodejs/ffi

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.28%. Comparing base (7946f82) to head (c49d330).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64998      +/-   ##
==========================================
- Coverage   92.02%   90.28%   -1.75%     
==========================================
  Files         399      762     +363     
  Lines      175608   247531   +71923     
  Branches    27107    46689   +19582     
==========================================
+ Hits       161609   223484   +61875     
- Misses      13687    15489    +1802     
- Partials      312     8558    +8246     

Files with missing lines	Coverage Δ
src/node_ffi.cc	70.93% <100.00%> (ø)

... and 492 files with indirect coverage changes

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