Fix default implementation on FixedVector (#2264)
Fix default implementation on FixedVector (#2264)

## Issue Addressed

NA

## Proposed Changes

Whilst hacking on something I noticed that the default implementation of `FixedVector` can violate the length constraint!

E.g., `let v: FixedVector<u8; U4> = <_>::default()` would create a fixed vector with length 0, even though it promises to *always* have length 4! This causes SSZ deserialization to fail and probably other things too.

This isn't a security risk as it can't be triggered externally, however it's a foot gun for LH devs.

## Additional Info

NA




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