Fix nil repositories in dev-cmd bump

[dduugg]

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Should resolve: https://github.com/Homebrew/homebrew-core/actions/runs/8364009822/job/22898307196#step:6:27

Possible regression: https://github.com/Homebrew/brew/pull/16887/files#diff-8d9528218ff2999bae098a7ce1556f42c61d546b285fa962ee62a8b2a35e9901R156

(Another option would be to widen the type signatures, which I prefer to avoid. That may be more semantically correct though, idk.)

[Bo98]

Wasn't that commit that regressed it because it was always conditionally nil but I agree nil arrays suck and this is better.

[MikeMcQuaid]

Thanks for quick turnaround here @dduugg and @Bo98!