CLN: no longer need to catch AttributeError, IndexError
cc @jreback @WillAyd 

As of today, we don't call `agg_series` unless `ngroups > 0`, which has the side-benefit that we don't call unless `len(self.bins) > 0`, which means we no longer risk hitting an `IndexError` in libreduction L262.

No longer needing to catch AttributeError is [mumble mumble...] I'll figure out what past-me was thinking after a good night's sleep.
thanks