Plugin fails in case of no changes

[u3r]

Hi,

thanks for the action! I encountered a small issue though.
I'm in the habit of creating release candidates before "full" release, which means if the release candidate actually works, I have no more changes.
This leads to something like No commits found between refs releases/2.3.0-rc1...HEAD

It would be cool, if

• either an EMPTY description would be created (I am ok with manually copying the last release notes over in that case)
• or the last one copied (probably too specific to my usecase)
  At least the plugin should not fail.