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

swayfx/sway/commands/corner_radius.c

Line 28 in 3e4b6b7

config->corner_radius = value;

which means that a corner_radius will always be set in the "global" configuration.

This PR adapts the check from

swayfx/sway/commands/blur.c

Lines 17 to 21 in 3e4b6b7

	if (con == NULL) {
	config->blur_enabled = result;
	} else {
	con->blur_enabled = result;
	}

[ErikReider]

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