Support query with no condition

[greenrobot-team]

Currently building a query requires specifying at least one condition. E.g. to use the upcoming query observer with an all objects query, this results in strange code like

final dummyCondition = dateProp.greaterThan(0);
_query = _box.query(dummyCondition)

Support building a query with no conditions (AFAIK at least OBX for Java allows this as well).

[Buggaboo]

Zum Beispiel:

// ...
// getter
Query get emptyQuery { /* return an empty query, skip the queryBuilder */ }
// ...

final query = _box.emptyQuery; // i.e. semantically: we want _everything_, no criteria, no conditions.
query.findStream();

Alternatively, what you also could provide is a stream of all (async) objects from a box, without all this ceremony.

final boxStream = box.stream;
final controller = StreamController<List<Entity>>();
controller.addStream(boxStream);

I'm working on relations, right now. This shouldn't be very hard.