Add that empty list can be returned in DistributionSupport docstring.
Add that empty list can be returned in DistributionSupport docstring.

PiperOrigin-RevId: 402848088
Change-Id: Ieeca77eeb95b9af8ad4cc14c9a59f612aadc0099

deepmind/open_spiel,deepmind/open_spiel,deepmind/open_spiel,deepmind/open_spiel,deepmind/open_spiel,deepmind/open_spiel