slices: tolerate Chunk([]string{}, 0) without raising panic for n<1

[mmetc]

I noticed many of my uses of Chunk have an optional value for n,
and it's ok to not split the data otherwise.

Unless the slice is empty, because if n is optional (0) the stdlib
implementation panics, so while migrating from my own implementation I
have to resort to an if/else or a contraption

for chunk := slices.Chunk(data, max(1, cmp.Or(size, len(data)))) {
...
}

which makes the code hard to follow and is prone to errors.

Tis PR keeps the panic for n<0 in all cases.

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