[ feature ] officially support 'empty' step

[lrozenblyum]

In different use cases gantt is used to display data by some categories (so each line represents a category).

In real life there might be no data for a particular category, while the line should be drawn anyway.

Now this behaviour can be unofficially simulated by

step.setStartDate( new Date() );
step.setEndDate( step.getStartDate() );

however it's a hack and looks not nice.

Could you add some official support for empty steps? Thanks