feat(cleanup-branches): input logic and error handling improvements

[ricky-undeadcoders]

This pull request updates the branch cleanup workflow to improve its handling of dry run logic and prevent failed pipelines when no branches qualify for deletion. It includes 2 changes:

Dry Run Logic Update

Swap the conditional logic for the "Delete branches (dry run)" and "Delete branches" steps in action.yml so that invalid entries will now default to dry-run. For example, consider the following circumstance:

uses: grafana/shared-workflows/actions/cleanup-branches
with:
    dry-run: falses

Currently we would delete the branches, since 'falses' != 'true'. The new logic sees that 'falses' != 'false' and does a dry run.

Handling for No Branches

Added a check to both branch deletion steps to skip execution and print a message if no branches are marked for deletion. Currently it will fail as there is no branches.txt file to parse.

[guicaulada]

I find it weird that dry-run is Opt-out instead Opt-in.

Why the choice to go with dry-run as the default?

I would expect:

- uses: grafana/shared-workflows/actions/cleanup-branches

Should just run the clean up branches action and do what it's intended, while now we need to specify:

uses: grafana/shared-workflows/actions/cleanup-branches
with:
    dry-run: false

Dry-run is usually a flag you must opt-in, not opt-out of.

[guicaulada]

Just realized this PR doesn't change this behavior, dry-run was already an opt-out flag since it was true by default, this just makes the behavior more consistent where an explicit, non-typo, false is required for it to opt-out of dry-run, which makes sense. My previous comment is still somewhat valid... but I don't think it's related to this PR anymore. 😅