Make it so pipeline operations on empty time series objects return a …

…series with a collection instead of having code unceremoniously wipe out.

pypond/pipeline.py

@@ -671,7 +671,14 @@ def to_keyed_collections(self):
             Returns the _results attribute from a Pipeline object after processing.
             Will contain Collection objects.
         """
-        return self.to(CollectionOut)
+        ret = self.to(CollectionOut)
+
+        if ret is not None:
+            return ret
+        else:
+            # return an empty dict so any calls to collection.get() won't cause
+            # things to unceremoniously blow up and just return None instead.
+            return dict()
 
     def to(self, out, observer=None, options=Options()):  # pylint: disable=invalid-name
         """

pypond/series.py

@@ -361,7 +361,14 @@ def set_collection(self, coll):
             New TimeSeries with Collection coll
         """
         res = TimeSeries(self)
-        res._collection = coll  # pylint: disable=protected-access
+
+        # pylint: disable=protected-access
+
+        if coll is not None:
+            res._collection = coll
+        else:
+            res._collection = Collection()
+
         return res
 
     def bisect(self, dtime, b=0):  # pylint: disable=invalid-name
@@ -1042,10 +1049,6 @@ def fill(self, field_spec=None, method='zero', fill_limit=None):
             the fill operation.
         """
 
-        # if there is nothing to fill, then just bail
-        if self.size() == 0:
-            return self
-
         pip = self.pipeline()
 
         if method in ('zero', 'pad') or \