Grouped mutation calls mutate before validation and skip empty groups #1141

This boils down to this loop:

for group in self._group_nodes(nodes).values():
    if not group:
        continue
    self._append_move_at(group, player, actions)

This does the append without checking everything's going to work OK, and empty groups are skipped silently. Meanwhile though, an empty list (ungrouped) raises an exception.

This issue will affect any by-group-enabled method, see #1140 as well.