Add validation in promotion workflow

[MarekMichali]

Description

Sometimes the curl where we fetch the latest release fails and we get null as a tag. Sometimes we also try to promote the same version that has already been promoted and as a result only 1 change (chart version) is pushed instead of 3 (chart version, app version and KEB version).

Changes proposed in this pull request:

• Check if the tag has been fetched and is not empty
• Check if there are 3 changes before committing

Related issue(s)

[MarekMichali]

/kind enhancement

[github-actions[bot]]

Add one of following labels

- kind/feature -> Use it when you want to submit a new feature

- kind/enhancement -> Use it when you modify or improve an existing feature

- kind/bug -> Use it when you fix a bug

[MarekMichali]

/kind enhancement