Called when evaluation of this filter query is about to start. The return value is passed
as a parameter to _includeRow and endQuery later. Returning a null value should mean that
_includeRow will return RES_EXCLUDE for all rows. So if you don't need a parameter object,
just return any object (this, new Object() or whatever) and simply ignore it later.
Return one of RES_INCLUDE, RES_EXCLUDE or RES_INGORE to indicate how to filter the
row. RES_INCLUDE means that the row should be included in the resurt except when
negation if query is desired. RES_EXCLUDE is the opposite, row is not included except
in the case of negation. RES_IGNORE means that row is not included regardless of query
being evaluated in negation mode or not. This can be because for example the association
doesn't have the assumed players.