Allow the standard deviation for GetSampleValuesForFrame to be 0

Setting the standard deviation to 0 is valid and should be interpreted
as directly using the sample value at the coordinates without weighting.
This is made explicit in the documentation for the Corruption Detection
extension:
http://www.webrtc.org/experiments/rtp-hdrext/corruption-detection

Also, change stddev to std_dev in halton_frame_sampler files.

Bug: webrtc:358039777
Change-Id: Id5aa4110194f7f2b2fe9914c94304c90afd64198
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/363300
Auto-Submit: Fanny Linderborg <linderborg@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#43070}