Set the truthiness value of BackgroundList

BackgroundList is expected to behave like a Python list. It is therefore natural to expect initializing an instance with no arguments to evaluate to False.

python/lsst/afw/math/_backgroundList.py

@@ -79,6 +79,9 @@ def __getitem__(self, *args):
     def __len__(self, *args):
         return self._backgrounds.__len__(*args)
 
+    def __bool__(self, *args, **kwargs):
+        return self._backgrounds.__bool__(*args, **kwargs)
+
     def append(self, val):
         try:
             bkgd, interpStyle, undersampleStyle, approxStyle, \