Fix feature extractor to be invariant to padding

[pzelasko]

What does this PR do ?

Adds tests and fixes inconsistency in ASR feature extractor when processing the same input with and without padding. Specifically:

• removes a Dirac-delta-like spike due to lack of masking in preemphasis (only present when audio_length < audio.shape["time"])
• replaces "reflect" padding with zero-padding for padding-length-invariance

I tested the changes on HF leaderboard with canary-180m-flash (offline model) and saw no significant change (actually the overall WER improved -0.01% to 7.11%).

This doesn't entirely solve the issue of numerical differences between padded and non-padded examples with Conformer models (especially offline ones), and needs follow-up on ConformerEncoder to fully resolve.

Collection: ASR

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