The designer will typically leave an extra row so that it would easier in the future to add more content in the section. When you preview, it shouldn't display that extra line if you do a couple of things in the designer: (1) place the cursor in the empty row and hit backspace (This will ensure there are no characters to display on the empty line (2) make sure the height of the section control is as much as the table inside it (you just need to grab the bottom anchor of the control and drag it up until it doesn't shrink anymore)
Hopefully that will remove the empty line for you when you preview the form.