Fix #11682: Screening plate rows with empty wells are too tall

When a row in a plate contains both images and empty wells, the row is too tall with extra whitespace below all the images

![screen shot 2013-11-14 at 10 18 52 am](https://f.cloud.github.com/assets/197099/1539704/dc72b770-4d10-11e3-8e87-8da92a3a921a.png)