Update release notes if tag exists

[mockitoguy]

Update release notes if tag already exists to better support teams that release on demand.

• Current behavior: githubRelease task performs the release, fails when the tag already exists (error is returned from GH API REST call).
• New behavior: if the tag already exists, and the release notes are empty, update the release notes.

Implementation note: it should work for the case when someone pushed a tag via Git push (in this case the release does not exist but the tag exists) or when someone created GH release via UI (release exists, tag exists).

[mockitoguy]

@shestee, do you want to take a stab at this?

[shestee]

@shestee, do you want to take a stab at this?

Yes, I will do it. Thank you for asking!

[mockitoguy]

Sample project: https://github.com/mockitoguy/shipkit-demo