fix masking issue in interpolate

[J22Melody]

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I have found this strange bug: when the x, y, z, and confidence are all 0s for a point in a pose (everything should be masked), but the current implementation on line 342 would not consider it masked when reading the first dimension (the x is 0).

I did not have this issue before, so I assume the pose data should always have masked x, y, and z values when the confidence is 0, @AmitMY is it what we should expect for a "normal" pose?

[AmitMY]

1. I am not sure i understand the change.
2. if confidence is 0, a mask is created https://github.com/sign-language-processing/pose/pull/161/files#diff-b2fa5baab13397a32417b3f212c9429c3398ddf14222e50a8eafa2d1af5f379fR50

[J22Melody]

ready for review and merge