Setting polarity to `"positive"` in `DetectAndMeasureConfig()` results in an error due to deblending

[peppedilillo]

Dears. Consider this snippet:

config = DetectAndMeasureConfig()
config.detection.thresholdValue = 5.
# the next line should select only positive sources but it's causing an error. maybe open an issue
# config.detection.thresholdPolarity = "positive"
results = DetectAndMeasureTask(config=config).run(calexp_injected, template, calexp_difference)

Setting thresholdPolarity to "positive" result in an error which seems to be due to the deblending task not checking for empty negatives.

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[120], line 5
      3 # the next line should select only positive sources but it's causing an error. maybe open an issue
      4 config.detection.thresholdPolarity = "positive"
----> 5 results = DetectAndMeasureTask(config=config).run(calexp_injected, template, calexp_difference)

File [/opt/lsst/software/stack/conda/envs/lsst-scipipe-10.0.0/share/eups/Linux64/utils/ge138a012be](https://data.lsst.cloud/opt/lsst/software/stack/conda/envs/lsst-scipipe-10.0.0/share/eups/Linux64/utils/ge138a012be)+fcb1d3bbc8[/python/lsst/utils/timer.py:307](https://data.lsst.cloud/python/lsst/utils/timer.py#line=306), in timeMethod.<locals>.decorator_timer.<locals>.timeMethod_wrapper(self, *args, **keyArgs)
    298 logInfo(
    299     obj=self,
    300     prefix=func.__name__ + "Start",
   (...)
    304     stacklevel=stacklevel,
    305 )
    306 try:
--> 307     res = func(self, *args, **keyArgs)
    308 finally:
    309     logInfo(
    310         obj=self,
    311         prefix=func.__name__ + "End",
   (...)
    315         stacklevel=stacklevel,
    316     )

File [/opt/lsst/software/stack/conda/envs/lsst-scipipe-10.0.0/share/eups/Linux64/ip_diffim/g216c3ac8a7](https://data.lsst.cloud/opt/lsst/software/stack/conda/envs/lsst-scipipe-10.0.0/share/eups/Linux64/ip_diffim/g216c3ac8a7)+b9c470ee50[/python/lsst/ip/diffim/detectAndMeasure.py:414](https://data.lsst.cloud/python/lsst/ip/diffim/detectAndMeasure.py#line=413), in DetectAndMeasureTask.run(self, science, matchedTemplate, difference, idFactory)
    412 results.positive.makeSources(positives)
    413 negatives = afwTable.SourceCatalog(self.schema)
--> 414 results.negative.makeSources(negatives)
    415 return self.processResults(science, matchedTemplate, difference,
    416                            results.sources, idFactory,
    417                            positives=positives,
    418                            negatives=negatives)

AttributeError: 'NoneType' object has no attribute 'makeSources'

I got this on the VM Weekly 2025_17 [f02e0429...] (sciplat-lab:w_2025_17) Medium (2.0 CPU, 8Gi RAM) data.lsst.cloud.

Hope that's useful.

Ciao,
Peppe