[fix] fix Sample when sampling only one element (fix #4134)

[shiyu1994]

This is to fix #4134. Currently, when K=1, it is impossible for the Sample function to sample the last element. Add a separate case for K=1.

[jameslamb]

this change makes sense to me! I read through all of https://github.com/shiyu1994/LightGBM/blob/fix-4134/include/LightGBM/utils/random.h and think I have a good understanding of this fix and enough C++/C knowledge to approve.

But I won't merge, in case you want another reviewer to also look.

[jameslamb]

@shiyu1994 do you want an additional review, or can this be merged?

[shiyu1994]

@jameslamb Sorry for the late response. I was busy with some developing work in the past few weeks. I think we can merge this.

[github-actions]

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