Providing a version for aggregating that is too high should aggregate to null

[erdtsieck]

// Given
var streamId = Guid.NewGuid();
var options = new StoreOptions();
options.Connection(ConnectionSource.ConnectionString);
options.Projections.LiveStreamAggregation<PolledAggregate>();

// When
var store = new DocumentStore(options);
var session = store.LightweightSession();
session.Events.Append(streamId, new PollingShouldFail(streamId));
await session.SaveChangesAsync();

// Then
var aggregate = await session.Events.AggregateStreamAsync<PolledAggregate>(streamId, 2);
aggregate.ShouldBeNull();