empty #O or #P list should not return empty string

[prjemian]

When the #On or #Pn control lines are parsed, if there are no positioners specified (see BCDA-APS/apstools#222 and aps-8id-dys/ipython-8idiuser#13), the result should be an empty list ([]).

Presently, #O0 results in [['']] and #P0 results in [].