Fix layout for subtitle/no subtitle scenarios.

Without a subtitle, there is a layout glitch when I compile the template ( 9a50b1ca ) with xelatex on my machine.

This merge request adds some space to handle presentations without subtitles better for now.

Before

With patch

With subtitle, before and after look equal to me.

changed the description

mentioned in issue #1 (closed)

assigned to @schryver

requested review from @schryver

added Bug label

merged

mentioned in commit ffbc2348