Make the python writers have the same behavior as the C++ writers

If no collections are passed then let the C++ decide instead of
passing all collections (which is what the C++ writers do). In practice,
this only allows to write empty frames from python, currently not possible
as collections will evaluate to false and all the collections from
`frame.getAvailableCollections` will be passed.

Author: jmcarcell

python/podio/base_writer.py

@@ -53,6 +53,7 @@ def write_frame(self, frame, category, collections=None):
                write. If None, all collections are written
         """
         # pylint: disable=protected-access
-        self._writer.writeFrame(
-            frame._frame, category, collections or frame.getAvailableCollections()
-        )
+        args = [frame._frame, category]
+        if collections is not None:
+            args.append(collections)
+        self._writer.writeFrame(*args)