global_move_bot_pr_to_board.yml should exit more gracefully when no bot PRs are open

[alyssadai]

Example: https://github.com/neurobagel/planning/actions/runs/7506808885

This workflow (1) looks for PRs with the _bot label applied and (2) processes them by moving them to the kanban board under "Community".

However, currently, when (1) does not find any matching PRs, (2) fails (expectedly) but this causes the entire workflow to fail.

We should see if we can modify the behavior so that the workflow exits without failing when (1) doesn't find any bot PRs.

[github-actions]

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