fix(core): allow shuffle to return first element in first index

[gracetxgao]

PR Checklist

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• The commit message follows our guidelines: https://github.com/graycoreio/daffodil/blob/develop/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

randomSubset with length 1 never returns the first element. This is because shuffle makes it impossible for the first element of the shuffled array to be the same as the first element of the original array, and randomSubset slices the shuffled array up to length to return the subset.

Fixes: #1780

What is the new behavior?

shuffle can now potentially return an array with the first element being the first element of the original array, so calling randomSubset with length 1 can return the first element.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[damienwebdev]

@gracetxgao this needs a rebase. It includes commits from #3056

Additionally, can you add a test covering the empty array case as well as the single-element case?

[damienwebdev]

lgtm