Allow for missing inputs when deblending

Sometimes inputs to the deblender are missing due to previous tasks
not producing them. This means the corresponding output catalog will
not be present. Adjust the 'put' logic in DeblendCoaddSourcesMultiTask
such that missing datasets will be set to None.

python/lsst/pipe/tasks/deblendCoaddSourcesPipeline.py

@@ -202,7 +202,10 @@ def runQuantum(self, butlerQC, inputRefs, outputRefs):
         sortedTemplateCatalogs = []
         for outRef in outputRefs.templateCatalogs:
             band = outRef.dataId['band']
-            sortedTemplateCatalogs.append(outputs.templateCatalogs[band])
+            # use the .get method because if the key does not exist, it will return
+            # None, putting None is the same as not putting something, and down-stream
+            # tasks can use that as a signal the dataset was not processed
+            sortedTemplateCatalogs.append(outputs.templateCatalogs.get(band))
         outputs.templateCatalogs = sortedTemplateCatalogs
         butlerQC.put(outputs, outputRefs)