Fix nullable-to-nonnull conversions for Xcode 26.6 (#20466)

[NSProgrammer]

Summary:

X-link: pytorch/pytorch#187979

Xcode 26.x errors under -Werror on implicit conversions from nullable NSString.UTF8String results to non-nullable const char * / std::string parameters. Guard each affected site with a ?: "" fallback (these paths are non-null in practice, so behavior is unchanged). Fixed sites (both the fbcode and xplat mirror copies):

• caffe2/.../mpscnn/tests/MPSCNNTests.mm: the sf.UTF8String append in the tensor-printing helper.
• executorch/.../ExecuTorchLLMTextRunner.mm: token.UTF8String (special tokens), _modelPath/_tokenizerPath (runner creation), and prompt.UTF8String (generate).
• executorch/.../ExecuTorchLLMMultimodalRunner.mm: _modelPath/_tokenizerPath (runner creation) and input.text.UTF8String (text input).

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Differential Revision: D109459540

[pytorch-bot]

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• [ROCm] MI350 CI jobs will have longer queue times due to CI migration

❌ 3 New Failures, 3 Unrelated Failures

As of commit 63b852d with merge base aada6d7 ():

NEW FAILURES - The following jobs have failed:

• pull / test-arm-backend-no-driver (test_pytest_ops_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t 7384ca1f283d30f01077e5f7f84d38f749ca265977fd26a6f669ec913d3c5ca1 /exec failed with exit code 1
• pull / unittest / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 88fa6ee190fdf582ad2df2caa6b84207b53f966356b430443965838023310f61 /exec failed with exit code 1
• pull / unittest-editable / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 59b00debb4837bc3424efc01d21caae75776d67cd7da45dcec41d4f9a5a7fd98 /exec failed with exit code 1

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh) (trunk failure)
• pull / unittest / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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[meta-codesync]

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