Guard NSString.UTF8String against nil in ExecuTorchModule.mm

[metascroy]

Summary:
Fix 8 sites in ExecuTorchModule.mm where -[NSString UTF8String] (returns
_Nullable) was passed directly into C++ Module APIs typed
const char *_Nonnull. On iOS Apple bindings these compile with
-Werror,-Wnullable-to-nonnull-conversion, so each site failed the build,
breaking messenger-jest-xcodestaging-iphonesimulator-{local,profile}-buck2
(298/280 consecutive failures since ~2026-05-12) and causing the iOS
pct_healthy regression tracked in T271188257.

Apply the standard fbobjc fallback pattern methodName.UTF8String ?: "" at
every affected call. Behavior is unchanged for any non-nil method name (the
common case); a nil/empty NSString now degenerates to an empty C string instead
of a null pointer, matching what the C++ API contract already required.

Affected call sites (all in
xplat/executorch/extension/apple/ExecuTorch/Exported/ExecuTorchModule.mm):

• line 322 _module->load_method(...)
• line 366 _module->load_method(..., cppMap)
• line 380 _module->is_method_loaded(...)
• line 384 _module->unload_method(...)
• line 407 _module->method_meta(...)
• line 552 _module->set_input(...)
• line 592 _module->set_inputs(...)
• line 635 _module->set_output(...)

The Messenger-owned sites in the same compilation are being addressed in
D105868578.

Differential Revision: D105904682

[pytorch-bot]

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❌ 1 New Failure

As of commit 4e5b024 with merge base 64684c7 ():

NEW FAILURE - The following job has failed:

• pull / unittest-editable / macos / macos-job (gh)
  backends/xnnpack/test/ops/test_conv2d.py::TestConv2d::test_fp16_conv2d

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[meta-codesync]

@metascroy has exported this pull request. If you are a Meta employee, you can view the originating Diff in D105904682.

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