Make VarZeroVec branchless like ZeroVec

[sffc]

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As discussed in https://github.com/unicode-org/icu4x/pull/5434#issuecomment-2332128845

To start, we should make VarZeroVec have private fields in 2.0. The actual optimization can be done later.

[Manishearth]

#5650 solves the 2.0 blocker here. The rest is gravy and doable in a backwards compatible way