[Android llama demo] Fix crash when tokenizer files are not found

[pytorchbot]

Return an array with size 0, to allow getting array length in code below.

[pytorch-bot]

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✅ You can merge normally! (1 Unrelated Failure)

As of commit a09ac48 with merge base 4b6331d ():

UNSTABLE - The following job failed but was likely due to flakiness present on trunk and has been marked as unstable:

• Android / test-llama-app / mobile-job (android) (gh) (#3344)
  Process completed with exit code 1.

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