Update audio_spec.py

[shaoyijia]

The preprocess_model used by BrowserFftSpec will produce "nan" results when the input tensors contain too many zero entries. Such inputs are common because there may be delays in the recording devices resulting in an empty stream at the beginning of the audio file. This PR fixes this potential "nan" problem by adding a small Gaussian noise to the inputs before they go through the preprocess_model.

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[shaoyijia]

@googlebot I signed it!