Search for _id always returns an iterator with an element

The element in the iterator will be null if the ID doesn't exist, but the iterator will say that there is an element.

		NitriteBuilder builder = Nitrite.builder();

		MVStoreModuleBuilder mvBuilder = MVStoreModule.withConfig();
		MVStoreModule mvModule = mvBuilder.build();

		builder.loadModule(mvModule);

		Nitrite db = builder.openOrCreate();

		try {
		
			NitriteCollection items = db.getCollection("items");
		
			Filter nameFilter = FluentFilter.where("name").eq("abc");
			DocumentCursor nameCursor = items.find(nameFilter, null);
			Iterator<Document> nameIter = nameCursor.iterator();

			assertFalse("no document matching name.", nameIter.hasNext());
			
			Filter idFilter = FluentFilter.where("_id").eq("0000000000000000000");
			DocumentCursor idCursor = items.find(idFilter, null);
			Iterator<Document> idIter = idCursor.iterator();

			assertFalse("no document matching ID.", idIter.hasNext());
		
		} finally {
			db.close();
		}