Fix eager loading bug: associations with primary key of 0 were not inclu...

...nded

isEmpty was set to true if identifier is 0 and the association was not loaded. Since Utils.firstValueOfHash returns null if the no value is present, we can do a === check to prevent 0 to be considered as false. (I know that 0  is a strange value for an integer primary key, but I do not have control on it in a particular project.)