I found a workaround, in cellTemplate you can check for an empty/null string and add the HTML  
(non breaking space) entity, this way your cell will be properly rendered.
Example:
cellTemplate: '<div class="yellow" ><div class="ngCellText">{{row.getProperty(col.field)||" "}}</div></div>'
Changed your plunker, here's a working example : http://plnkr.co/edit/tR6UKCrXZqKi5HwsWOAV?p=preview