ssz, tests: fix slice init on empty decode

[karalabe]

Fixes https://github.com/karalabe/ssz/issues/18.

The issue was that decoding an zero-length dynamic array into a nil slice left the destination slice as nil, which isn't inherently bad, but it's a bit wonky since it should ideally become a zero length array, signalling that there's "no data" vs. "missing field". That does require an allocation, but feels better than leaving nil.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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Additional details and impacted files

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