Unused `background` arg in `characterizeImageTask.run`

[danjampro]

Hi - I've come across what appears to be an unused arg in characterizeIamgeTask.run:

pipe_tasks/python/lsst/pipe/tasks/characterizeImage.py

Line 443 in 0c551f4

def run(self, exposure, exposureIdInfo=None, background=None):

Specifically the background input is not used, and the variable is reassigned here:

pipe_tasks/python/lsst/pipe/tasks/characterizeImage.py

Line 482 in 62ad60b

background = self.background.run(exposure).background

[laurenam]

Looks like the check for a supplied background got lost in this commit: 1a9814c