I have the feeling that this feature doesn't work anymore, or I don't understand how it's working.
I have a content with 2 fields : body, and media field.
Media field is optional.
I put media field in left column and body in middle column.
I checked "hide empty regions"
If in the content page the media field hasn't been provided, the div of the left column is displayed anyway, apparently $left is not empty
<div class="field field-name-field-video-event field-type-media field-label-hidden"><div class="field-items"></div></div>
i'm using the last dev version of 22th september