[SYCL][Graph] Fix recording in_order queue with empty nodes

[mfrancepillois]

Adding a empty node to a recorded in-order queue resulted in inconsistent dependencies between nodes.
This patch fixes this issues and simplifies the adding of empty nodes.
Unitests have been added to check node dependencies when recording an in_order queue with and without empty nodes.

Fixes Issue: #239

[EwanC]

Looks great, always a nice patch when removing code fixes an issue.

[Bensuo]

If I'm not mistaken this change also changes the default behaviour of the handler when Graphs are not involved which I'm not sure about. Now an empty command group is passed to the scheduler instead of doing nothing and just returning an empty event.

The potential for unnecessary overhead there concerns me, and in general I think we should avoid modifying the default behaviour unless it is necessary or it improves it in some way.

[mfrancepillois]

If I'm not mistaken this change also changes the default behaviour of the handler when Graphs are not involved which I'm not sure about. Now an empty command group is passed to the scheduler instead of doing nothing and just returning an empty event.

The potential for unnecessary overhead there concerns me, and in general I think we should avoid modifying the default behaviour unless it is necessary or it improves it in some way.

Thanks for your review.
You are right, we should avoid sending empty node to scheduler in default mode.
I modified the code patch to keep the default behaviour as it was.

[Bensuo]

LGTM, just some nitpicks!