If you specify an integration branch that doesn't exist on the remote, the plugin fails, with the following error:

Failed to commit merged changes. Error message below
 (nothing to squash)Already up-to-date.
HEAD detached at xxxxxxx
nothing to commit, working directory clean

This is because it does a git checkout -B <integration_branch>
If the specified integration branch doesn't have a matching branch on the remote, git checkout will happily create a new local branch pointing to the current HEAD without complaining. This is normal and expected git behaviour, but in this case has the obvious side-effect that the resulting merge is empty, as it will be merging from HEAD (the ready branch) to a new branch pointing to the same commit as HEAD.

As it doesn't really make sense to integrate to a non-existing integration branch, situations like this should be considered configuration errors. The plugin should somehow check that the integration branch exists.