FIlling space with empty bounding box

I'm detecting objects on images. I want to detect up to 10 objects, however, I'm not sure how to deal with the situation, where only one object is present.

Should I fill the remaining spaces in the label input data with vectors filled with 0? E.g:

[[xmin,ymin,xmax,ymax],[0,0,0,0]...]


Or is there any better way? Thanks for help!