FrameSVG documentation states
> ... topleft and topright will be ignored if top does not exist,
> and similarly for bottomleft and bottomright ...
Someone would assume that the same behaviour applies to the case
when left(or right) does not exists.
This change makes FrameSVG to comply with such expected behaviour.
### Before
{F5878439, layout=center, size=full}
The bottom border doesn't exist so the bottom-right corner should not
be drawn.
### After
{F5878440, layout=center, size=full}