ci(Check cherry-picks): gracefully fail when original commit hash cannot be identified

[LeSuisse]

Description of changes

Follow-up to this report: #172098 (comment)

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[risicle]

Hmm.. I think we should probably try and avoid processing (and emitting any output) for any commits that don't look like cherry-picks, both for speed and clarity. The reason it was breaking is just because my recent method of using <<< "$(...)" for the iteration will emit an empty string when no matching commits are found.

[LeSuisse]

Fine by me, it less likely to generate unwanted failures 👍 . Let's go for #303211