Guard NSString.UTF8String against nil in ExecuTorchModule.mm #14

Job	Run time
test-riscv (mobilebert, true, true) / run / linux-job	1d 0h 0m 0s
test-riscv (mobilebert, true, false) / run / linux-job	1d 0h 0m 1s
test-riscv (resnet18, true, false) / run / linux-job	1d 0h 0m 1s
test-riscv (mv2, true, false) / run / linux-job	1d 0h 0m 0s
test-riscv (llama2, true, false) / run / linux-job	1d 0h 0m 0s
test-riscv (add, true, false) / run / linux-job	1d 0h 0m 0s
test-riscv (resnet18, false, false) / run / linux-job	1d 0h 0m 0s
test-riscv (mv2, false, false) / run / linux-job	1d 0h 0m 1s
test-riscv (mv2, true, true) / run / linux-job	1d 0h 0m 1s
test-riscv (mobilebert, false, false) / run / linux-job	1d 0h 0m 1s
test-riscv (add, false, false) / run / linux-job	1d 0h 0m 0s
test-riscv (llama2, true, true) / run / linux-job	1d 0h 0m 1s
test-riscv (resnet18, true, true) / run / linux-job	1d 0h 0m 1s
test-riscv (llama2, false, false) / run / linux-job	1d 0h 0m 1s
	14d 0h 0m 8s

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