Only apply corner_radius to container if there is one

[itsCryne]

I noticed that when I use a for_window command in my sway config where I set the corner_radius, this applies to every window as soon as a window matching the for_window-criteria was opened.

This seems to happen due to a missing check at https://github.com/WillPower3309/swayfx/blob/3e4b6b75f4b42b4cb04d883033b04b4a1b57bc90/sway/commands/corner_radius.c#L28
which means that a corner_radius will always be set in the "global" configuration.

This PR adapts the check from https://github.com/WillPower3309/swayfx/blob/3e4b6b75f4b42b4cb04d883033b04b4a1b57bc90/sway/commands/blur.c#L17-L21

[ErikReider]

We'll have to wait with this until #367 is merged :)