Fix WhisperNoSpeechDetection when input is full silence

[ylacombe]

What does this PR do?

@cifkao found an edge case that happens when the input of Whisper.generate is a full silence. This is a simple tentative PR.

cc @sanchit-gandhi

Fixes #29036

[ylacombe]

I want to add a test, but I realized most of the slow tests of Whisper were already failing, independently of this PR. cc @sanchit-gandhi

[HuggingFaceDocBuilderDev]

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[cifkao]

Seems to work for me!

[ArthurZucker]

Do you need a review @ylacombe ?

[ylacombe]

I do, thanks !