TypeError: <Target> object is not iterable when check_complete(dep) is False

[GianlucaFicarelli]

When a Task is run, and one of the dependencies is not fulfilled, and this dependency has a single Target as output (instead of a list of targets, or a dict of targets), then Luigi fails with:

Traceback (most recent call last):
  File "/usr/local/lib/python3.10/site-packages/luigi/worker.py", line 188, in run
    nonexistent_outputs = [output for output in dep.output() if not output.exists()]
TypeError: 'RemoteTarget' object is not iterable

The failing code is at this line and it seems related to the changes in #3258.

I think that the code should be fixed to handle the case when dep.output() isn't a list of targets.